Under water photon communication by single photon detection

ABSTRACT

An optical underwater communication system is disclosed which includes a first transceiver and a second transceiver, each including one or more optical sources configured to provide light activated and deactivated according to a first bit stream, one or more sensor packages each comprising a plurality of photodetectors configured to receive light from the other transceiver and, in response, provide an output voltage signal and an output current signal, a detector configured to i) convert the output voltage signal and the output current signal to pulses associated with arrival of photons, and ii) count the number of pulses based on a predetermined timing sequence, an encoder configured to encode a message to be sent into a first bit stream, and a decoder configured to decode a message received into a second bit stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 63/327,758, filed Apr. 5, 2022 and entitled “SINGLE PHOTON BASED LIFE-TIME SPECTRAL DETECTION CYTOMETRY AND SORTING,” further claims priority to, and the benefit of, U.S. Provisional Patent Application No. 63/327,759, filed Apr. 5, 2022 and entitled “UNDER WATER PHOTON COMMUNICATION BY SINGLE PHOTON DETECTION,” and is further a continuation-in-part application of U.S. Non-Provisional patent application Ser. No. 17/536,068, filed Nov. 28, 2021, entitled “PHOTON COUNTING AND SPECTROSCOPY,” which is a continuation application of a US nationalization application Ser. No. 16/604,557, filed Oct. 10, 2019 entitled “Photon Counting and Spectroscopy” from an international application No. PCT/US18/27302, filed Apr. 12, 2018 entitled “Photon Counting and Spectroscopy,” which claims priority to, and the benefit of, U.S. Provisional Patent Application No. 62/485,207, filed Apr. 13, 2017 and entitled “Photon Counting and Spectroscopy,” each of which is incorporated in its entirety herein by reference.

STATEMENT REGARDING GOVERNMENT FUNDING

None.

TECHNICAL FIELD

The present disclosure relates to a system for underwater communication, and in particular to a system and a method used therein suitable for underwater communication via single photon detection.

BACKGROUND

This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.

The most common communication technique for submarines is acoustics. The primary reason for this type of communication is because it is essentially transparent in water. However, these signals can be captured by any detector in the region as acoustic signals travel in water by virtue of waves. Thus, another submarine or a listening device by an over-water vessel can eavesdrop and thus present a dangerous situation.

Acoustic wave used, presently underwater, offer low transfer rates (in the order of kilobits per seconds). However, modern network and video communication protocols require communication several order of magnitude larger, in the range of Megabits pers second or even Gigabits per second which cannot not satisfied by acoustic wave.

Thus, there is an unmet need for a system that can replace acoustic underwater communication without the chance of being detected by an eavesdropping device.

SUMMARY

An optical underwater communication system is disclosed which includes a first transceiver and a second transceiver. Each transceiver includes one or more optical sources configured to provide light activated and deactivated according to a first bit stream, one or more sensor packages each comprising a plurality of photodetectors configured to receive light from the other transceiver and, in response, provide i) an output voltage signal corresponding to photoelectron response of an incident photon on the one or more sensor packages, and ii) an output current signal in form of peaks corresponding to photoelectron response of an incident photon on the one or more sensor packages, a detector configured to i) convert the output voltage signal and the output current signal to pulses associated with arrival of photons, and ii) count the number of pulses based on a predetermined timing sequence, an encoder configured to encode a message to be sent into a first bit stream, according to a predetermined communication protocol that is based on the predetermined timing sequence, and a decoder configured to decode a message received into a second bit stream, according to the predetermined communication protocol that is based on the predetermined timing sequence.

An optical underwater communication method is also disclosed. The method includes activating and deactivating light by one or more optical sources according to a first bit stream by a first transceiver and a second transceiver, each transceiver receiving light from the other transceivers by one or more sensor packages each comprising a plurality of photodetectors and, in response, provide i) an output voltage signal corresponding to photoelectron response of an incident photon on the one or more sensor packages, and ii) an output current signal in form of peaks corresponding to photoelectron response of an incident photon on the one or more sensor packages, converting the output voltage signal and the output current signal to pulses associated with arrival of photons by a detector, counting the number of pulses based on a predetermined timing sequence by the detector, encoding a message by an encoder to be sent into a first bit stream, according to a predetermined communication protocol that is based on the predetermined timing sequence, and decoding a message by a decoder into a second bit stream, according to the predetermined communication protocol that is based on the predetermined timing sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects, features, and advantages of various aspects will become apparent when taken in conjunction with the following description and drawings. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures. The attached drawings are for purposes of illustration and are not necessarily to scale.

FIG. 1 shows a block diagram of an example excited-photon spectrum measurement/analysis system.

FIG. 2A shows an example structure of a silicon photomultiplier (SiPM) circuit.

FIG. 2B shows example characteristics of an SiPM.

FIG. 2C is an elevational cross-section of an example SiPM.

FIG. 2D shows electric-field characteristics of an example SiPM.

FIG. 3A is a graphical representation of a micrograph of an example SiPM.

FIG. 3B is a circuit diagram of an example SiPM.

FIG. 4A shows an example SiPM readout circuit including a differentiator, and related SiPM components.

FIG. 4B shows measured data of an output from the circuit of FIG. 4A for a single photon pulse.

FIG. 4C shows measured data of an output from the circuit of FIG. 4A for multiple photon pulses.

FIG. 5 shows an example differentiator circuit usable, e.g., in the circuit of FIG. 4A.

FIG. 6A shows measured data of an example input to the differentiation stage of the circuit of FIG. 5 .

FIG. 6B shows measured data of an example output from the differentiation stage of the circuit of FIG. 5 , for the input of FIG. 6A.

FIG. 7 shows an example adaptive comparator circuit.

FIG. 8 shows example inputs and outputs of the adaptive comparator circuit of FIG. 7 .

FIG. 9 shows a block diagram of signal processing for photon detection, and related components.

FIG. 10A shows an example circuit configuration of an adaptive pedestal clamping circuit.

FIG. 10B shows another example circuit configuration of an adaptive pedestal clamping circuit.

FIG. 11A is a graphical representation of a micrograph of an example SiPM.

FIG. 11B depicts characteristics of longitudinal multiplication in an SiPM.

FIG. 11C depicts characteristics of transverse multiplication in an SiPM.

FIG. 12 shows simulated inputs and outputs of the circuit of FIG. 13 .

FIG. 13 is a circuit diagram of an example filter section in an envelope-detector circuit.

FIG. 14 is a circuit diagram of an example pulse-width counter/discriminator circuit.

FIG. 15 shows example timing diagrams of the circuit of FIG. 14 .

FIG. 16 shows an example spectrofluorometer configuration according to various prior schemes.

FIG. 17 shows a single-photon spectrometer configuration according to various examples herein.

FIG. 18 shows sample-illumination and resultant-light-collection optics according to various examples herein.

FIG. 19 shows some components and optical paths of an example motorized monochromator.

FIG. 20 shows an example configuration of an optical system permitting wall-less fluorescence detection of liquid samples by coaxial illumination in a tube.

FIG. 21A shows a spectrum spread spatially, and example fiber-bundle shapes.

FIG. 21B depicts components and operation of an example monochromator.

FIG. 21C shows a cross-section illustrating dense packing of optical fibers.

FIG. 21D shows an example circular fiber bundle.

FIG. 21E shows another example circular fiber bundle.

FIG. 21F shows an example rectilinear fiber bundle.

FIG. 21G shows another example rectilinear fiber bundle.

FIG. 22 shows an end view of an example using a relatively large-core fiber and a slit aperture at the end of the fiber.

FIG. 23 shows components of an example optical system supporting on-axis and perpendicular measurement.

FIG. 24 shows an example flow-cytometry system according to various examples herein.

FIG. 25 shows an example data-processing system according to various examples herein.

FIG. 26A shows a perspective of components of an example motorized monochromator.

FIG. 26B shows an elevational cross-section of an example polychromator.

FIG. 27A shows an example linear fiber array and connector.

FIG. 27B shows output fibers fed by a polychromator.

FIG. 28 shows an example SiPM linear array.

FIG. 29 shows components of an example optical system and array.

FIG. 30 shows an example system for high-throughput sample measurement.

FIG. 31 is an isometric view of tubes in a portion of a plate array such as that in FIG. 30 .

FIG. 32 shows a plan view of an example silicon microelectromechanical system (MEMS) photomultiplier (PM).

FIG. 33 shows an elevational cross-section of the PM of FIG. 32 .

FIG. 34 shows an example photon-counting measurement system, e.g., using a PM.

FIG. 35 shows circuitry of an example silicon photomultiplier.

FIG. 36 shows circuitry of an example silicon photomultiplier including a fast readout electrode.

FIG. 37 shows an equivalent circuit illustrating characteristics of the photomultiplier of FIG. 36 .

FIG. 38 shows a graph of electromagnetic attenuation in water based on wavelength.

FIG. 39A shows a sensor package previously shown in FIG. 4A which includes a number of Geiger mode avalanche photodiodes each coupled to a quench resistor; the sensor package is shown conceptually coupled to downstream circuits.

FIG. 39B shows a schematic utilizing the sensor package shown in FIG. 39A coupled to downstream circuits and alternative software paths.

FIG. 40A shows a conceptual graph of amplitude vs. time of amplified raw signals from the sensor package of FIG. 39B and an adaptive threshold provided to a comparator and the output of the comparator.

FIG. 40B shows an alternative adaptive threshold comparison scheme employed in software where the amplified raw signals from the sensor package of FIG. 39B is digitized, and level-shifted by an envelope of the negative side of the signal.

FIG. 40C shows the alternative adaptive threshold comparison scheme of FIG. 40B compared with a threshold, where not only are peaks detected but also peak widths, and border between peaks are also determined.

FIG. 40D shows a conceptual graph of amplitude vs. time where both alternative approaches shown in FIG. 39B and further depicted in FIGS. 40A, 40B, and 40C applied to the same amplified raw signals from the sensor package of FIG. 39B depicting the same digital comparison outcome.

FIG. 40E shows a comparison of the number of counts per second acquired by the photon detector or photomultiplier between comparator with and without adaptive threshold schemes.

FIG. 41 is a schematic showing two transceivers each including a transmitter and a receiver depicting a scheme to align the transceivers for improved communication distance.

FIG. 42 shows a schematic of a proportional-integral-derivative (PID) controller that can be used to control the components shown in FIG. 41 .

FIG. 43 shows graphs of attenuation coefficient (1/m) vs. wavelength (nm) which can be used to depict the attenuation vs. distance (m) based on various absorption coefficients (e.g., 0.01, 0.025, and 0.05).

FIG. 44 shows an optical schematic at each transceiver with a light detection and ranging (LIDAR) function.

FIG. 45 shows a graph of beam diameter (expressed as spot size) in mm vs. distance also in mm.

FIG. 46 shows a quadrant sensor for XY alignment scheme.

FIG. 47 shows a graph of attenuation distance based on Beer-Lambert law for underwater communication.

FIG. 48 shows a graph of laser photon number vs. laser power for a laser having a wavelength of 450 nm, utilizing different technologies.

FIG. 49 shows a schematic of distance measurement scheme for underwater optical communication applications according to the present disclosure.

FIG. 50 shows a schematic of raster scanning, which shows two transceivers 1 and 2, where transceiver 1 can detect direction and distance of transceiver 2 by reflective beams.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.

In the present disclosure, the term “about” can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range.

In the present disclosure, the term “substantially” can allow for a degree of variability in a value or range, for example, within 90%, within 95%, or within 99% of a stated value or of a stated limit of a range.

A novel system is disclosed herein that can replace acoustic underwater communication without the chance of being detected by an eavesdropping device. Towards this end, a novel system including a sensor package comprising Geiger mode avalanche photodiodes is disclosed which operates on the basis of detecting single photons as a form of underwater communication scheme.

Water absorbance is very high in the visible region. Sea water is particularly important for communication between submarines and ships. Sea water is particularly transparent to blue-green spectrum.

Thus, visible light especially in the blue-green region offers lower absorption in water. Typically, light emitting diodes (LEDs) or laser modulation are applied in optical communication. On the other hand, modulated light intensity rapidly decreases by water absorption and scattering of particles. Consequently, detection sensitivity is critical for extending communication distance.

Others have used photomultiplier tubes (PMTs) for underwater communication. The PMTs are used in electrical current mode with significant challenges. For example, in electrical current based detection technologies utilizing PMTs, in order to achieve a square wave pulse at least a 100 to 1000 photoelectron is required. Photoelectrons are electrons which are emitted from a material when the material is stuck by electromagnetic radiation, e.g., light. However, even applying high sensitivity PMTs, only a distance in the range of 100 m is possible for underwater communication. Thus, extending communication distance is another goal of the system of the present disclosure. In addition, laser communication requires in-line alignment between transmitter and receiver in 3D space. Thus, an on-axis tracking module is needed between the transmitter and receiver to ensure proper alignment.

To reduce unnecessary complexity, the system of the present disclosure requires only incident photon to generate a pulse. These photons and their associated pulses can then be used to establish an underwater communication scheme which is immune to eavesdropping devices as the photons are transmitted from point to point and can be implemented with substantial reduction in complexity as compared to PMTs.

An actual reduction to practice has shown that the system of the present disclosure can create a signal and transmit through water, and return that signal accurately. The system additionally utilizes an encoding and decoding approach to produce an effective signal transmission.

The photodetectors utilized according to the present are typically Geiger mode avalanche photodiodes. Photodetectors generate electrons when excited by photons of the emitted light from a source. These excited electrons generate a signal which can be measured and decoded as a message sent from a transmitter and received by a receiver. Common light sources includes lasers. Common lasers include ultraviolet (UV) having a wavelength of 355 nm to 360 nm, violet having a wavelength of 405 nm to 407 nm, blue having a wavelength of 488 nm, red having a wavelength of 633 nm, yellow having a wavelength of 561 nm, and green having a wavelength of 532 nm. Blue-green laser is found to be the most effective in seawater.

The disclosure provided below often refers to flow cytometry, a field where light having a first wavelength incident on a particle of interest accompanied with a fluorescent molecule can cause electrons in the fluorescent molecule to be energized to a higher energy level from a ground state and when the electrons relax back down to the ground state, the electrons release electromagnetic radiation in the form of emitted light at a second wavelength, different than the first wavelength. Photodetectors can be used to detect the emission from the fluorescing molecules and thereby obtain information about the particle of interest. However, it should be understood that many aspects of underwater communication based on single photon detection is only tangentially related to flow cytometry only insofar as in both fields, according to the present disclosure a photodetector sensor package is disclosed comprising Geiger mode avalanche diodes which can be used with very short dead time to generate pulses associated with individual photons striking the sensor package, regardless of whether the photons are emitted from a fluorescent molecule or from a source of light in an underwater communication scheme.

Accordingly, various aspects relate to a silicon (Si)-based photon sensor and excited photon spectrum analyzer, e.g., a differential Geiger-mode photon spectroscope. Various examples receive photons at a solid-state photodetector that provides an output signal, then pass the output signal through a differentiator to provide a signal representative of the number or rate of photons striking the photodetector. Various examples count pulses or pulse widths of this representative signal to determine the rate of photons striking the photodetector. Various aspects can be used in flow cytometry, but are not limited thereto. Various aspects provide increased sensitivity and dynamic range compared to prior schemes. Various aspects relate to Single Photon Spectroscopy, e.g., counting each individual photon striking a photodetector. Various aspects can provide increased sensitivity for hyperspectral analysis, compared to prior schemes.

Laser induced fluorescence (LIF) analysis can be performed by photon counting for basic materials in flow cytometry, microscopy, imaging, and material science. Auto fluorescence (AFL) and photo bleaching phenomena in quartz, glass, sheath fluid, or other biological materials can be measured based on total photon counting over detectable spectral ranges. In order to investigate low level fluorescence, single photon spectral analysis with high spectral resolution can be performed in the spectral domain (i.e., across various wavelengths of light). Various aspects herein include devices, systems, and methods for performing single photon spectrometry. Various aspects use a motorized monochromator and a photon detector as described herein. Various aspects can measure the phenomena described in this paragraph, or optical characteristics of glass, quartz, or other light-transfer or -blocking materials, e.g., materials used in optical filters, lenses, or other components; in optically-clear vessels or adhesives; or in optical stops.

Various aspects permit using single-photon detection in flow cytometry, microscopy, or imaging. Various examples permit using single-photon detection in physics and chemistry experiments, e.g., the evaluation of materials such as optical components or other materials. Other examples of materials can include liquids, e.g., reagents, fluorochromes, buffers, or consumable liquids such as oils, wine, beer, juice, or water. Various examples of material evaluation permit measuring properties of materials with higher sensitivity and speed than some prior schemes, and at multiple wavelengths of light. Some examples permit evaluating properties of carbon nanotubes or other nanostructured materials. Such properties can include perhaps absorbance, autofluorescence, or reflectance. Some examples include measuring such properties using techniques described herein, and comparing the measured properties to references (e.g., in a database) to determine qualitative characteristics of the materials. Such characteristics can include, e.g., whether the materials are genuine.

In a tested example, samples were illuminated by 405-nm laser light in a 100 μm-diameter spot (after a bandpass filter) to reduce or eliminate accelerated spontaneous emission (ASE). Excited fluorescence on the optical axis was collected by a NA 0.125 lens and coupled to NA 0.22 optical fiber (after a laser wavelength notch filter). In some examples, e.g., using a filter with an optical density of about six (OD6), an additional notch or long pass filter can be used to separate laser illumination and induced fluorescence. The optical fiber was connected to a motorized monochromator having a 500 nm/sec scanning speed over a 200-900 nm spectral range. A synchronized stepping motor was used to drive a reflective grating. Photoelectrons were counted in a preset gating time for each spectral step selected. This permitted measuring at spectral steps of 1 nm and time intervals (gating times) of 0.1 sec. Counted photoelectrons were measured at specific photon energy levels. Such measurements can be performed in the spectral domain or the time domain. In some examples, spectral steps can be between ˜0.1 nm and ˜10 nm. In some examples, gating times can be between ˜1 ms and ˜10 s.

Various examples permit measuring or characterizing AFL spectra and bleaching phenomena, e.g., of basic materials used in flow cytometry instruments. Some examples permit observing spectra such as Raman spectra excited by laser wavelength, impurities, and molecular structures in materials. Single photon spectroscopy as described herein can provide increased sensitivity and wide dynamic range compared to prior schemes. Some examples can permit quantitative analysis of fluorescence, which can permit investigation of material characteristics as well as bio assays that exhibit lower levels of fluorescence than do assays testable by some prior techniques.

Various examples include a silicon (Si) photon sensor and single photon spectrometer that provide high sensitivity and permit quantitative analysis for fluorescence using relatively short measurement times. Single photon spectroscopy can permit next-generation cellular analysis and material improvement. Some examples can collect samples in microsecond-scale time windows via parallel spectral detection.

Various aspects relate to a wide-dynamic-range Si Photon Detection System using a differential Geiger-mode sensor. Digital photon detection based on the Einstein-Planck equation can permit sensing small numbers of photons. However, for photodetection in the context of biology, e.g., flow cytometry, some prior photodetectors do not provide a sufficiently wide dynamic range to effectively capture conjugated fluorescence signals. Therefore, various flow-cytometry schemes use photocurrent detection from a photomultiplier tube (PMT), solid-state photomultiplier (PM), or micro-PMT.

Various examples herein provide a Si photon sensor and electronics with a wide dynamic range, e.g., six orders of magnitude of dynamic range. Accordingly, sensors herein can be used in cytometry or other biological-analysis contexts.

Some examples use arrayed Si avalanche photodiodes (APDs) operating in the Geiger mode. Incident photons on a pixel (e.g., a particular APD) produce electron-hole pairs. The corresponding current is magnified by several orders of electrons by breakdown of the electric field at p-n junctions. Charged electrons are quenched by a serial register in each pixel, e.g., in ˜50 ns for a ˜100 kΩ quench resistor and a ˜500 fF pixel capacitance. In prior schemes, the quenching time of a pixel defines the upper limit of photon pulse counting per second. The lower limit is determined by dark count or dark count deviation caused by thermal effects. In general, the dynamic range of a multi-pixel Si photomultiplier is dependent on the number of pixels.

Various examples herein collect a differential signal from a pixel. This style of collection is referred to herein as “differential Geiger mode.” Using differential signals permits detection of multiple photons from a pixel even within the quenching time of that pixel. Shorter pulse pair resolution (less than 1 ns), as in various examples, can permit resolutions up to, e.g., one billion counts per second (1 Gcps). Various examples include a very-high-speed comparator and GHz-rate counter electronics, which can permit detecting and counting sub-ns photon pulses. Various examples have a low dark count of 50 kcps/mm² at room temperature, in a tested configuration. Various examples provide further reduction of the dark count by actively cooling the sensor, e.g., using a cooling unit. Various examples include a Peltier cooling unit and can achieve dark counts lower than e.g. 1 kcps, or about 100 cps at −30° C., with 10 cps standard deviation. Various examples include system-control and measurement software.

Various example Si photon detection systems herein can evaluate from sub-fW to nW range light power with theoretical linearity. Various examples provide a detectable sensitivity improved by about 3 orders of magnitude compared with prior photocurrent detection schemes. Various examples provide picosecond time resolution, which can permit measurement or analysis of individual molecular interactions in live cells. Some examples permit calibrating light power and analyzing spectrum as a measurement technology. Some examples include, e.g., calibrating based on knowledge of the Si photon sensor photon-detection efficiency (PDE) and reflective grating diffraction efficiency for particular wavelengths. Various examples can permit measuring intrinsic auto fluorescence phenomena of optical and biologically-pertinent materials such as quartz, glass, pure water, or silica or polymer beads.

Various examples can measure individual photons, which may have very low energies. Values of photon energy versus wavelength for typical laser wavelengths in flow cytometry are: 405 nm: 740 THz 3.06 eV; 488 nm: 614 THz 2.54 eV; 532 nm: 563 THz 2.33 eV; 594 nm: 504 THz 2.08 eV; 633 nm: 473 THz 1.95 eV; 780 nm: 384 THz 1.58 eV. Owing to the small energy per photon, the number of photons per pW can be expressed as megacounts per second (Mcps), e.g.: 405 nm: 2.04 Mcps/pW; 488 nm: 2.46 Mcps/pW; 532 nm: 2.68 Mcps/pW; 594 nm: 3.00 Mcps/pW; 633 nm: 3.20 Mcps/pW; 780 nm: 3.95 Mcps/pW. In general, 1 pW is the lowest detection limit of a PMT photocurrent signal. In general, a photon sensor has thermal noise, known as dark count, in the range of 1 cps to ˜1 Mcps. Dark count is sensitive to temperature and determines the detection limit. In addition, dark-count standard deviation per second (r) and coefficient of variation CV (%) are considered as the resolution limit of light intensity. Temperature control of the sensor can improve dark count and its standard deviation.

Photon counting is the digital measurement of light intensity with extremely high sensitivity and linearity. If a detected photon pulse is an ideal impulse with pulse width “zero” and dark count “zero,” the photon detection system is ideal. Unfortunately, real photon pulses have finite pulse width and waveform. The upper count rate is determined by pulse width and the lower limit by dark-count rate. Owing to pulse overlapping, the true count value N and measured value M are described as: N−M=NMt, where t is the pulse pair resolution. For example, with t=1 ns, the error is relatively small up to 10 Mcps and a gradually larger error for higher count rates. If necessary, it is possible to correct the measured value up to ˜1 Gcps by this model. 1 Gcps is equal to 0.5/QE nW at 405 nm and 0.25/QE nW at 780 nm (QE=quantum efficiency). Therefore, it can be possible to achieve over six decades of magnitude and linearity with dark count <1 kcps and pulse pair resolution 1 ns, e.g., at a maximum 1 Gcps measurement. However, photon pulse pair resolution in some prior schemes is longer than 10 ns. Furthermore, measured signals have fluctuations that produce counting threshold deviation, causing additional error in some prior sensors.

Various examples can reach these levels of performance Dark count rate (DCR) is another factor to determine sensitivity limit. In general, dark count is proportional to sensor area. A smaller photocathode or sensor active area usually reduces dark counts. Reducing sensor area by 90% may reduce dark count by about 90%. However, prior schemes, e.g., of flow cytometer optics for fluorescence detection, have relatively large aberrations as well as a large spot image due to broad wavelength and high NA collection lens without compensation. Various examples use reflective optics or optical fiber coupling to improve detection with smaller sensors.

Sensor structure and material can also significantly contribute to dark counts. Dark-count origins include thermal noise in the sensor or photocathode. Materials with higher sensitivity in the IR region have higher dark-count characteristics. For example, comparing bialkali and multi-alkali photocathode materials for detection at extended longer wavelengths, multi-alkali shows a higher dark count. Various examples control dark count and reduce signal deviations by temperature control. Peltier cooling can be used to reduce dark count. For cooling purposes, a smaller sensor is easier to implement.

Light intensity measurements with theoretical linearity can be performed by photon counting in the digital domain. Various examples permit analyzing the photon spectrum or photon energy. Hyperspectral analysis is a useful technique for cellular analysis. Photon spectroscopy can be implemented in at least two ways. One is in combination with a motorized monochromator and photon detector using a long capture time (>1 s). A recent motorized monochromator has a wavelength scanning speed of 500 nm/s. Because photon measurement for flow particles is in the us to ms time domain, some examples include a parallel photon detection system.

Various examples permit biological fluorescence analysis using detectors having high sensitivity and wide dynamic range with linearity in wavelengths of interest, e.g., visible wavelengths or other wavelengths. Photon detection has sufficient sensitivity if dark count is low. The upper dynamic range of photon detection is mainly determined by maximum count capability per second. Various examples provide a shorter photon pulse width and reduced dead time, compared to some prior schemes.

Throughout this description, some aspects are described in terms that would ordinarily be implemented as software programs. Those skilled in the art will readily recognize that the equivalent of such software can also be constructed in hardware, firmware, or micro-code. The present description is directed in particular to algorithms and systems forming part of, or cooperating more directly with, systems and methods described herein. Aspects not specifically shown or described herein of such algorithms and systems, and hardware or software for producing and otherwise processing signals or data involved therewith, can be selected from systems, algorithms, components, and elements known in the art.

Steps of various methods described herein can be performed in any order except when otherwise specified, or when data from an earlier step is used in a later step. Example method(s) described herein are not limited to being carried out by components particularly identified in discussions of those methods. Section headers are solely for ease of reference and do not restrict the combinations covered by this disclosure. A feature described in a particular section can be used in conjunction or combination with features described in that section or in other sections.

Illustrative Configurations, Techniques, and Operations

FIG. 1 shows a block diagram of an example excited-photon spectrum analyzer system, and an example graph (lower left) of output in counts per second (cps) as a function of wavelength in nanometers (nm) from 400-800 nm. The example graph shows a visible range of 400 nm-800 nm, but this is not limiting. Various examples use a differential Geiger mode for photon detection. Example detectors can include an array of avalanche photodiodes having capacitive couplings (“C-couplings”) to a sensor output (a “fast output”). In some examples, the fast outputs are wired in parallel, e.g., as shown in FIG. 36 . Example photon sensors, and related components, are described herein with reference to FIG. 2A-15, 19, 21A-22, 27A-28, 32, 33 , or 35-37. For example, any of the following SENSL SiPMs can be used: 10010, 10020, 10035, 10050, 30020, 30035, 30050, or 60035. For example, an SiPM can have an active area of 1 mm², 3 mm², or 6 mm²; a photodiode-cell size between 10 μm² and 50 μm²; a fill factor between 25% and 75%; a photodiode count between 250 and 3000; or a photodiode density between about 100 mm⁻² and about 1000 mm⁻². “BP” is a bandpass filter; “notch” is a notch filter. “LaserBP” is a bandpass filter that will substantially pass the laser light; “Laser notch” will substantially block the laser light. “Giga-counter” is an electronic pulse counter, e.g., capable of counting at GHz rates.

The laser light L, or other electromagnetic radiation radiated into the sample, is referred to herein as “incident light.” Light or other electromagnetic radiation transmitted through the sample, or light emitted from targets, fluorescent dyes (e.g., bound to targets), or other substances within the sample, is referred to herein as “resultant light.” Resultant light can include electromagnetic radiation provided by, e.g., scattering, refraction, absorption, or rotation of the plane of polarization of the incident light, or any combination thereof. Throughout this document, the terms “scatter,” “scattering,” and the like include refraction, absorption, and polarization rotation, as appropriate, unless otherwise explicitly noted. Resultant light can include forward-scattered (FS) light and side-scattered (SS) light. FS and SS have substantially the same wavelength as the light source, e.g., the laser. Resultant light can also include fluorescent light, since such light can be emitted by the sample or substances within the sample. Resultant light can be substantially directional (e.g., transmitted light of the laser light L) or substantially omnidirectional (e.g., fluorescence), or can have a selected or characteristic radiation pattern (e.g., a cardioidal variation of intensity as a function of angle from the direction of the incident light).

Various example sensors or detection systems herein provide at least one of: a fast output, an amplifier having a bandwidth >200 MHz, an impedance <200Ω, or a pulse width (pulse duration) of <5 ns. For example, bandwidth >200 MHz and impedance <200Ω can permit detecting 1 ns-wide pulses. In some examples, sensors as described herein, e.g., with reference to FIG. 2A-4A, 11A-11C, 32, 33 , or 35-37 can be used as the illustrated “photon sensor.” Various examples of photon-counting spectral-analysis systems such as that shown in FIG. 1 can provide or include pulse clamping (e.g., FIG. 9, 10 , or 1517), a comparator (e.g., FIGS. 7 and 9 ), or very-high-speed counting (e.g., FIG. 9 ).

Various example systems and techniques shown in FIG. 1 and elsewhere herein can permit collecting data and analyzing the data to determine, e.g., wavelength or time dependencies for phenomena such as photo bleaching. Various examples permit detecting photon counts and, from those photon counts, together with at least one of photon energy, illumination power, optics NA, or sensor PDE, quantitatively estimating and analyzing fluorescence characteristics. Various examples permit determining optical-filter performance, performing material AFL analysis, performing color analysis, performing bio-assay analysis, or performing cellular or particle analysis.

In the illustrated example, incident light from the laser strikes the sample. Resultant light is collected in the optical fiber and carried to a spectral-discrimination device (“spectral device” or “spectro unit”), e.g., a monochromator or polychromator. The illustrated example shows a “motor driven spectro unit,” e.g., a motorized monochromator. However, this is not limiting. In other examples such as some shown below in Table 1 and FIGS. 26B-29 , a motorized or non-motorized polychromator can be used, e.g., with an array sensor or a fiber array. The spectral-discrimination device permits measuring different wavelengths of light separately. In FIG. 1 , a single photon sensor successively measures each wavelength of interest in the resultant light. In other examples, one or more photon sensor(s) or elements thereof can concurrently measure respective wavelength(s) of the resultant light. The measurements can include rates of photon incidence on the photon sensor, e.g., in counts per second.

In various examples, a photon sensor can include a sensing element such as a vacuum-tube based photomultiplier tube (PMT), a solid-state (e.g., Si) photomultiplier tube (SSPMT, e.g., FIGS. 32 and 33 ), a silicon photomultiplier (SiPM, e.g., FIGS. 2A-3B), an avalanche photodiode (APD), or a single-photon avalanche diode (SPAD). The term “tube” is used in reference to SSPMTs merely to distinguish them from SiPMs and other types of photon sensors. The term “tube” does require, that an SSPMT have any particular pressure (e.g., vacuum) or material contained in the SSPMT. In various examples, a photon sensor can include multiple sensing elements, e.g., an array of SSPMTs arranged on a printed circuit board (PCB), or an array of SiPMs, APDs, or SPADs arranged on a wafer (e.g., FIGS. 2A-3B). In some examples, a photon sensor can include combinations of any of these. SSPMTs, SiPMs, APDs, SPADs, and arrays of any of those are examples of solid-state photon detectors (SSPDs). The term “SiPM” is used for brevity and can include other semiconductor photomultipliers (SPMs) that include APDs or SPADs, e.g., APDs or SPADs on InGaAs or SiC substrates, or other semiconductor types discussed below. In some examples, SiC is used for detection of ultraviolet photons.

Various examples can use different spectral-discrimination devices or photon sensors. Nonlimiting example combinations can include those listed in Table 1. Throughout this document, “CH” refers to a channel of optical measurement. Examples of various components listed in Table 1 are described herein with reference to FIG. 1-6B, 9, 11A-11C, 17, 18, 19, 20, 21A-21G, 22, 23, 24, 25 , or 26A-29.

TABLE 1 Example Number of Spectral light photon Type of photon Example features of Device extraction sensors sensor and mount some aspects Monochromator Angular scan 1 Any SSPD listed wide λ range(200- across slit, above (e.g., 900 nm); Accurate optical fiber FIGS. 1 or 26A) FWHM; Flexible coupling, or combination sensing element Polychromator Linear scan 1 Any SSPD listed λ range (340~800 nm); across fiber above, receiving simple mechanism; coupling light via an optical faster scan than fiber (e.g., FIG. angular; no 2^(nd) 26B, linear scan) harmonic Polychromator Linear scan 1 Any listed above, As above; All in one; across mounted to compact; lower cost sensing receive light element (e.g., directly from mounted polychromator SSPD) (e.g., FIG. 26B, array) Polychromator No 32-64 SSPD array Parallel detection; movement; sensor, or group of High speed detection fixed fiber discrete sensing in μs array, 32- elements. 64CH Mounted to receive light from respective optical fibers (e.g., FIGS. 27A and 27B Polychromator No 128-256 SSPD array As above; Low dark movement; (or more, sensor, mounted to count; All in one fixed SiPM e.g., 1024 receive light array or 2048) directly from polychromator (e.g., FIG. 26B, array)

FIG. 2A shows an example structure of a SiPM, which is an example of a solid-state photon detector (SSPD). An SiPM such as that shown can provide small size, high quantum efficiency, lower bias voltage, light durability, insensitivity to magnetic fields, lower cost, and more, compared to a conventional PMT. In a PMT, an incident photon produces an electron-hole pair in the photocathode, which is an electrical insulator; vacuum and high voltage can permit capture of the induced electron. On the other hand, the electron-hole pair of a solid-state sensor such as that shown is produced in the p-n junction, which is semi-conductive material. The produced electron moves rapidly and the acceleration depends on the reverse bias electric field. The illustrated SiPM includes a photodiode (PD), e.g., an APD or SPAD, and a quench resistor R, discussed below.

FIG. 2B shows various examples of principles of operation of example SiPMs. In the Geiger region of operation, the bias voltage is sufficiently high in magnitude (e.g., >5×10⁵ V/cm) that, when a photon strikes the sensor and releases a photoelectron, that electron can strike other atoms and release additional electrons. Accordingly, a single photon can trigger a cascade of electrons that causes the SiPM to become conductive and produce a detectable current. There are three phases of operation: P-I-N mode with gain=1, linear avalanche mode with gain ˜100 and Geiger mode over break-down voltage with gain ˜10⁶. Geiger mode is highly sensitive for incident photons owing to a high QE ˜0.8 and gain >10⁶.

Once a photon has hit and triggered an avalanche, a quenching resistor in series with the PD is used to reset the PD without requiring active circuitry. As the current increases, the quenching resistor's voltage increases, and so the photodiode's voltage decreases. Once the current increases to the point that the photodiode's voltage is below the Geiger breakdown threshold, conduction will substantially cease until the photodiode's voltage has risen above the breakdown threshold and the next photon impact event takes place. Quenching time, typically 50 ns, is called dead time because the detector will not fire even if struck by a photon. In order to expand the dynamic range, a Geiger mode sensor has a structure arrayed as pixels (FIG. 3A). SiPM photon detection efficiency PDE is defined as QE×ϵ_(geo)×ϵ_(trig), where ϵ_(geo) is the geometrical fill-factor and ϵ_(trig) is the avalanche triggering probability. When a pixel is “fired,” a SiPM has secondary fire phenomena called “cross-talk,” at adjacent pixels, and “afterpulse,” a delayed signal in the fired pixel. Various examples permit performing accurate photon-count measurements even in the presence of these factors.

FIG. 2C shows an elevational cross-section of an example SiPM fabricated on a wafer.

FIG. 2D shows an example of electric-field magnitude during operation of an SiPM.

APD cells vary in dimension from 20 to 100 microns depending on the mask used, and can have a density of up to 1000/mm². Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high-speed telecommunications using optical fibers, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe (Mercury Cadmium Telluride) operates in the infrared, to a maximum wavelength of about 14 μm, uses cooling to reduce dark currents, and can achieve a low level of excess noise.

Silicon avalanche diodes can function with breakdown voltages of 100 to 2000V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 100-200 V in silicon). Greater voltage can be applied to silicon APDs, which are more sensitive compared to other semiconductor photodiodes, than to traditional APDs before achieving breakdown allowing for a larger operating gain, preferably over 1000, because silicon APDs provide for alternative doping. Reverse voltage is proportional to gain, and APD gain also varies dependently on both reverse bias and temperature, which is why reverse voltage should be controlled in order to preserve stable gain. SiPMs can achieve a gain of 10⁵ to 10⁶ by using Geiger mode APDs which operate with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level.

Geiger-mode APDs produce relatively large charge pulse when struck by a photon of the same amplitude no matter the energy of the photon. When reading out conventional APDs, noise of the preamplifier significantly degrades timing and amplitude resolution performance for short (shorter then ˜500 ns) light pulses. Compared to conventional APDs, SiPMs using Geiger mode APDs provide much higher output amplitude, which can reduce the effect of preamplifier noise.

FIG. 3A is a graphical representation of a micrograph, and FIG. 3B is a circuit diagram, showing example configurations of SiPM arrays. As shown, a sensor can include rows and columns of sensor elements. As shown in FIG. 3B, each sensor element (or individual sensor elements) can include a quench resistor in series with a Geiger-mode avalanche photodiode (APD), e.g., a SPAD. as in Example details of these components are shown in FIGS. 2A-2D. The sensor elements (“pixels” or “microcells”) can be electrically in parallel across a row, column, or 2-D sensor array. As current flows through the APD in response to impact of a photon thereon, voltage across the quench resistor increases. Therefore, voltage across the APD decreases. When the voltage across the APD drops below the Geiger threshold (FIG. 2B), the APD will cease to conduct and will be ready to detect another photon. This process is referred to as “quenching.”

An array SiPM (e.g., using Geiger mode PDs) such as that illustrated can provide high gain (e.g., >10⁶) and high sensitivity of photon detection. Such sensors can be relatively compact and can operate with relatively low bias voltages, e.g., <25-70 V. Such sensors can be durable under light exposure and can be relatively insensitive to magnetic fields. However, some prior schemes have a limited dynamic range due to the limited number of pixels on the sensors and the dead time during quenching, during which those prior sensors do not detect photons. Moreover, some prior sensors have relatively high dark count rates (i.e., appreciable output signal even when no light is incident) or can be sensitive to temperature variations.

In some examples, the SiPM avalanche process is very fast, e.g., on the order of picoseconds (e.g., 10 ps-100 ps). However, the quenching (“recharge” or “dead time”) process is comparatively slow, e.g., on the order of nanoseconds (e.g., 50 ns-100 ns). As noted above, during some prior schemes, the sensor does not detect photons during the recharge process. Accordingly, some prior sensors only provide a dynamic range of about three orders of magnitude.

Some examples sensors and detection systems herein permit using SiPMs for biological photon detection applications that require wider dynamic range, e.g., six orders of magnitude. Various examples detect the avalanche mode using a combination of a high pass filter and a differentiation signal processing block. A tested example was able to perform multiple photon detection, even during the dead time. Accordingly, various examples can provide a dynamic range up to, e.g., six orders of magnitude

FIG. 4A shows an example circuit using a differentiator (represented as an RC high-pass filter followed by a buffer (triangle)) to provide 1 ns photon pulses. In some examples, the differentiator produces a time derivative (e.g., ΔV/s or AA/s) of the input signal (e.g., V or A). Using a differentiator can permit detecting multiple photons using a single pixel, which can in turn increase dynamic range. The SiPM output waveform has a fast avalanche process measured in picoseconds and a slow recharge process measured in nanoseconds. The high-pass filter and signal differentiation permit detecting only the avalanche process. Experiments were performed using an ultrahigh-speed differentiating circuit with GHz-bandwidth amp or equivalent differentiation in pixels to show multiple photon detection even during quenching dead time. This measurement technique, and corresponding signal processing described herein, are referred to for convenience and without limitation as “differential Geiger mode.” In some examples, a monolithic or on-chip capacitor is used to perform filtering with reduced parasitic capacitance and inductance compared to off-chip parts. In the illustrated example, the differentiator's input (on capacitor C) is connected opposite the quench resistor from the APD. The illustrated differentiator can additionally or alternatively be connected to: the anode of the SSPMT shown in FIGS. 32 and 33 ; the Vbias or Iout electrodes shown in FIG. 35 ; or the Vbias, Fast out, or Iout/Common electrodes shown in FIG. 36 .

FIG. 4B shows an example output from the circuit of FIG. 4A, showing that a single pulse produces a detectable spike in the differentiated photon signal output by the differentiator circuit.

FIG. 4C shows an example output from the circuit of FIG. 4A. As shown, the differentiated photon signal shows pulses even when a photon strikes the detector during the dead time (quenching period). Although the differentiator is shown as attached on the anode side of the APDs, it can additionally or alternatively be attached on the cathode side of the APDs. Similarly, the quench resistor for an individual APD can be attached on either the anode or the cathode side of that APD.

An experiment was performed to measure a photon pulse using an SiPM without a fast out, e.g., similar to the configuration of FIG. 4A. A unipolar pulse of FWHM ˜600 ps duration was detected, with a pulse height of ˜50 mV.

An experiment was conducted to measure counts per second (cps) from a sensor as a function of the power of the light incident on the sensor in pW. The experimental results showed a positive correlation between pW and cps. That correlation was substantially linear in certain regimes, e.g., 0-400 pW and 600-1400 pW. In some examples, accordingly, counts above 250 Mcps can be provided and counted. An experiment was also conducted to measure signals output by a differentiator circuit, e.g., as discussed herein with reference to FIGS. 4 and 5 . The experimental data showed discernable pulses.

FIG. 5 shows an example differentiator circuit including a high-pass filter section and a differentiation section. Various example ranges of component values are illustrated. The values of the components illustrated can be adjusted, e.g., to provide differentiation signal processing with a rise time T_(rise) corresponding to the timescale of the avalanche process with respect to a particular SiPM, e.g., 10 ps-100 ps. In some examples, differentiation circuitry such as that shown is used with SiPMs or similar devices. Differentiation circuitry can be used to increase detection sensitivity, e.g., of photon detectors that have asymmetric waveforms (e.g., ps-range rise times and ns-range fall or quenching times).

In some examples, differentiation signal processing can distinguish photons within overlapped photon pulses. This can provide more accurate photon counting, since photons that would otherwise be missed during an SiPM's dead time can be detected. In the illustrated circuit, an ideal square pulse produces a Gaussian pulse after the high pass filter. Circuit parameters of the differentiation section can be adjusted to provide separation of two partially-overlapping Gaussian pulses. Some example circuit parameters can include:

-   -   High-pass filter: C_(HP) (Farad)=T_(RISE)/50     -   Differentiation filter: C_(DIFF) (Farad)=(⅕)CHP=T_(RISE)/250     -   Primary corner frequency: F0 (Hz)=1/(10 T_(RISE))     -   T_(RISE): avalanche process time; typical <100 ps

FIG. 6A shows an example input to the differentiation stage of the circuit of FIG. 5 . FIG. 6B shows an example output from that circuit. As shown, the differentiation stage responds to two overlapping pulses (top), e.g., triggered by two separate photons, by providing a signal having two zero-crossings (bottom). Other thresholds than zero can additionally or alternatively be used. As indicated by the double-ended arrows in the top plot, a multiple-photon pulse can have a wider pulse width (longer duration) than a single-photon pulse. Accordingly, a pulse width differential can be used to discriminate closely overlapped 1-photon pulses from 2-photon pulses. Examples of pulse discrimination are described herein with reference to FIG. 14 .

FIG. 7 shows an example adaptive comparator circuit. The example circuit can provide, e.g., adaptive reference (threshold) level changes following pulse modulation. Due to AC-coupling of outputs from the differentiator section, large excursions of the signal baseline can occur among rapid successive pulses. To reduce miscounts, the illustrated circuit causes threshold level(s) respond adaptively by tracking the signal baseline. An RF envelope detector, e.g., monolithic or made from discrete COTS components, produces a signal that closely follows the negative peak excursions of the input. An appropriate filter further smooths the signal. A constant offset is added to the envelope to produce the tracking threshold level(s). The pulse is delayed, e.g., through at least one of a long transmission line or a passive network, so that the pulse arrives to the comparator at the same time as the moving threshold(s).

FIG. 8 shows example inputs and outputs of the adaptive comparator circuit of FIG. 7 . Two thresholds are illustrated: a large-pulse threshold and a small-pulse threshold. In some examples, the small-pulse threshold is used to detect single-photon excitation (1PE) pulses and the large-pulse threshold is used to detect double-photon excitation (2PE) pulses. A 2PE pulse can result from near-simultaneous absorption of two photons, or from absorption of a single photon of a shorter wavelength.

FIG. 9 shows a block diagram of signal processing for photon detection, and related components. Various examples are designed to operate with at least one of: an avalanche process ˜100 ps, a photon pulse width <1 ns, or a quenching time >50 ns. In some examples, output can be taken from, or referenced to, the cathode, the anode, or both. Although shown as connected on the anode side of the APDs, the measurement electronics can additionally or alternatively be connected on the cathode side.

FIG. 9 shows sequencing of signal processing. FIG. 5 shows differentiation before a comparator. FIG. 7 shows determination of the comparator threshold. FIG. 9 shows an example overall flow of signal processing: photo current-HP-Pedestal clamping-differentiation (FIG. 5 )-comparator (FIG. 7 ).

In some examples, the differentiation section can mix the signal with the differential of the signal. This can accentuate overlapping peaks, increasing dynamic range.

In some examples, a pedestal correction factor between 0.1 and 0.15 can be used. In some examples, the pedestal correction can be determined based on the light level (e.g., high vs. low) using feedback from the adaptive threshold determining unit.

In some examples, a full amplifier is used. In some examples, e.g., in which capacitance is implemented in each sensor pixel, a pre-amplifier (preamp) can be used without a full amplifier.

An experiment was conducted to measure single-pulse and multiple-pulse detection. In the tested example, multiple photons were detected, even during the avalanche quenching time. Some prior schemes are unable to detect photons during the quenching time.

In some examples, the high pass filter permits detecting avalanche processes as <1 ns-width pulses, e.g., corresponding to a single photon. In some examples, the high pass filter output can be processed to detect multiple photons even during a recharge process, unlike prior schemes.

FIGS. 10A and 10B show example circuit configurations of adaptive pedestal clamping circuits, e.g., as discussed herein with reference to FIG. 9 . In some examples, the illustrated circuitry can stabilize the pedestal level of short photon pulse signals. In some examples, as a number of incident photons increases, a signal pedestal fluctuates due to the high-pass filter. However, counting photon signals without the high-pass filter can lead to level deviations and a smaller signal, which can result in the limited dynamic range of some prior schemes. Therefore, some examples count the high-pass filter or differentiator output and use pedestal clamping circuitry to reduce pedestal-level variation and to stabilize the inputs to the comparator.

Some examples improve the accuracy of the pedestal clamping by reducing the phase difference between the pixel-signal output and the differentiator (“HF”) output. For example, components of the circuit can be adjusted, or delay lines or other delay elements added, to keep the phase shift of the electronics below the pixel pulse width.

An experiment was conducted to measure data of pixel output signals, differentiator (“HF”) output signals, and signals after adaptive pedestal clamping. Tests were performed at a relatively lower count rate (lower optical power incident on the sensor) and at a relatively higher count rate. The tested adaptive-pedestal circuit provided a steady baseline from which a threshold for determining counts could be set. In some examples herein, once the baseline has been set by the adaptive pedestal clamping, the threshold determined by the adaptive threshold unit can be applied to count pixels by counting times when the differentiator output crosses the threshold.

In some examples of a Gaussian profile having a full width at half maximum (FWHM) of 1.2 ns, pulse height and pulse resolution are negatively correlated. For a photon pulse amplitude following a Gaussian form, comparator pulse resolution is correlated with count accuracy. In some examples, to detect 1 ns pulse widths, 200 ps pulse resolution is used. Reducing the pulse resolution can reduce the measured count. Some commercial universal counters have a pulse resolution of 4 ns, and some 1 GHz digital oscilloscopes have a resolution of about 0.4 ns. Therefore, in some examples, a high-frequency counter, e.g., implemented using an FPGA, is used to provide, e.g., 200 ps pulse resolution or better resolution.

FIG. 11A is a graphical representation of a micrograph of a portion of an example SiPM.

FIGS. 11B and 11C show examples of estimation of the avalanche process time, and internal structures of example SiPMs.

FIG. 12 shows simulated outputs of the circuit of FIG. 13 . The SPICE simulation conditions were. tran 0.00001 0; mark=1000000000; space=0; PULSE (1.5 0.5 0.000002 0.00000001 0.00000001 0.000002 0.000004).

FIG. 13 shows an example filter section in an envelope-detector circuit (e.g., FIG. 7, 8, 10A-10B, 13 , or 14). The illustrated filter section can be used, e.g., in the adaptive threshold circuit of FIG. 7 . In some example envelope detectors, the diode is specified for GHz operation with junction capacitance, e.g., about 1 pF-about 10 pF. Some example envelope detectors include a transimpedance amplifier (TIA). Component values can be, in some examples: V2 1.4 V; L1 22 nH; R3 50Ω; C1 22 nF; R4 50Ω; R1 50Ω; C2 20 pF; R2 5 kΩ; C3 20 pF.

FIG. 14 shows an example pulse-width counter/discriminator circuit. Some examples include various ways to distinguish pulse widths. In general, a fixed comparator threshold level can be used to detect pulse width difference. As an example, the block diagram shown in FIG. 14 can discriminate between single- and overlapped-photon pulses.

In some examples, illustrated circuits can detect two overlapping photon avalanches that result in a pulse that is longer (wider) than a “standard” pulse width of a single-photon pulse. For linearity of the measurement system, some examples identify such pulses as being two photons rather than one photon.

The illustrated examples use two comparators: Comp 1 and Comp 2. The comparators can be single-ended- or differential-output. Various comparator technologies can be used, e.g., technologies that allow for propagation delays of hundreds of picoseconds.

FIG. 15 shows example timing diagrams of the circuit of FIG. 14 . In the illustrated examples, longer pulses are counted twice: once with the lower counter on Comp 1, and once with upper counter on Comp 2. The total count is then the sum of the lower counter's count and the upper counter's count. In illustrated examples, Comp 2 can only go high if the pulse is longer than one standard pulse-width, because Comp 2 is only enabled once a standard pulse-width's worth of time has passed after the onset of the pulse. This can prevent double-counting of single photons while still providing accurate counting of two overlapped photon pulses.

In some examples, Comp 1 is continuously enabled while measurements are being taken. Comp 2 is enabled by Comp 1, after a delay to. The input signal is also delayed by an amount less than to, so that it arrives at Comp 2 one standard pulse-width before Comp 2 becomes enabled. Therefore, Comp 2's input signal is high when it becomes enabled only if the pulse is wider than one standard pulse-width. In some examples, the buffer and delay line introduce a delay of t_(d), the predetermined standard pulse-width). The propagation delay t_(d) is the time between when the input pulse arrives at Comp 1 and when comp 2 becomes enabled (t_(d)<t_(o)). The standard pulse-width varies by device and preamp. In some examples, the standard pulse-width can be ˜400 ps-˜900 ps. The standard pulse-width can be selected empirically and embodied physically, e.g., in delay lines, or as a stored configuration value in software or firmware. For a long pulse, Comp 2 outputs a high level and stretches the output pulse in time, so that the counter can register the pulse.

The illustrated polarity is an example and is not limiting. For example, the counters can increment on an active-low or active-high clock. Suppose a positive rising pulse is applied to IN+ and a threshold is applied to IN−. The comparator will go LO→HI as the positive pulse crosses the threshold. This is appropriate for an active-high counter. For a negative pulse, the inputs would be flipped in order to continue using an active high comparator. In some implementations there may be multiple buffers or multiplexers for signal observation and sourcing; those muxes could invert the signal one or more times.

FIG. 16 shows an example spectrofluorometer configuration according to various prior schemes. Various prior schemes suffer from at least one of the following limitations. Use of an incoherent white light source (e.g., a xenon or halogen lamp) and two monochromators can result in large power consumption for the lamp, e.g., 150 W; low excitation power through a slit; intensity that is difficult to measure; or difficulty observing high-intensity phenomena like photo-bleaching. A perpendicular optical layout between the sample and a fluorescence detector, used to avoid effects due to the excitation light, limit the configuration and shape of samples and sample cells that can be used. Use of an emission monochromator and photocurrent detection can prevent the use of quantitative collecting optics, resulting in measured intensity values in arbitrary units rather than meaningful photometric units, and can provide a low sensitivity due to photocurrent detection. Accordingly, some prior schemes do not provide quantitative fluorescence measurement, and can be bulky and expensive.

FIG. 17 shows a single-photon spectrometer configuration according to various examples herein. Illustrated is a coherent light source, e.g., a laser. Other lasers or sources that provide one or more specific wavelengths of light can be used. An illumination intensity control unit can be used, and can include, e.g., at least one of a laser power control unit, a polarizer, or a neutral density (ND) filter.

In some examples, illumination optics can include a lens (L); a laser notch filter (NF), e.g., a dielectric filter, at a beam waist of the laser beam or other incident light; and a laser bandpass filter (BP) between L and NF. A sample (SL) or other target to be measured can be located adjacent to or otherwise proximal to the NF filter. This can provide a double excitation effect by reflection of the laser beam off the notch filter. Examples are discussed herein, e.g., with reference to FIG. 18 or 20 . Fluorescence detection optics downstream of SL and NF can include additional notch filters, long-pass (LP) filters, lenses to focus emitted resultant light, e.g., fluorescence, to an optical fiber, or any combination thereof. A motorized monochromator can select a particular wavelength of light to direct to the photon sensor. The photon sensor and associated electronics, e.g., as in FIGS. 1-15 , can drive a counter to count pulses. The counter value can be read by, e.g., a personal computer (PC) and software.

In some examples, meta-material absorption materials can be used to reduce effects due to stray light in a spectrometer. For example, a carbon-nanotube absorber can have <0.1% reflectivity. This can increase the dynamic range of the measurement system.

Some examples use an optical bandpass filter (BPF). In various examples, a BPF can reduce or substantially eliminate accelerated (or amplified) spontaneous emission (ASE). In some examples, an incident laser beam induces ASE photons with a broad spectrum, e.g., red-shifted slightly with respect to the incident light. In an example, the ASE peak extends from approximately 430 nm to approximately 465 nm. To improve the accuracy of photon counting, ASE can be reduced by adding a bandpass filter, e.g., centered at 405 nm (or other laser peak wavelength) with a width of, e.g., 20 nm (e.g., 395 nm-415 nm). Data can be measured of stray light and autofluorescence at various power levels, demonstrating that the measurement system can effectively characterize even the performance of some of its own optical components.

In a tested example, a SiPM was illuminated with a light source through a cover having a thickness of ˜3 μm. The measured data exhibited peaks at specific wavelengths, indicating that interferometric effects were present in the resultant light being measured. These effects can be due to protective layers on the SiPM die or package. To reduce noise due to such interferometric effects, anti-reflective (AR) coating(s) can be used on the SiPM or optical elements associated therewith.

Additionally or alternatively, detectors as described herein can be used to measure layer thicknesses using interferometric effects. An example optical structure can include two interfaces (“1-2” and “2-3”) between three layers (“1,” “2,” and “3”). In some examples, at least one of the layers (e.g., layer 1, or layers 1 and 3) can be air. For example, Layer 2 can represent a cover of the SiPM. The thickness d of Layer 2 can be determined based on the wavelengths (nm) of the peaks and valleys of the interference pattern (λ_(n)), and on a refractive index n. In an example, layer 1 is air (n≈1.0), layer 2 is a transparent layer having refractive index n, and layer 3 is a reflective layer, e.g., comprising silicon. Using measurement techniques herein can permit, e.g., measuring the thickness d of the cover of the SiPM (layer 2) using diffraction patterns. In various examples, detection circuitry as described herein (e.g., SiPM sensors and circuitry such as shown in, e.g., FIG. 9 ) can permit measuring thickness of optical filters or other elements, e.g., imaged onto a cover-less SiPM (or a SiPM with an AR-coated cover).

FIG. 18 shows illumination and collection optics according to various examples herein. “NF” is a notch filter. “FL” is fluorescence. Various examples permit readily aligning the laser beam and the filter/sample positions. In some examples, the laser beam is focused on the NF filter to obtain a conjugate image, i.e., an image that follows an optical path symmetrical to the optical path of the incident illumination. In some examples, the laser beam is reflected by the NF and excites the sample again. This provides a double excitation effect and roughly double the detection sensitivity. For example, if the reflected laser beam illuminates a portion of the sample that has not yet been illuminated, another fluorescent photon may be emitted.

In some examples, the collection optics can have a consistent numerical aperture (NA). Therefore, the collection efficiency can be determined quantitatively. Moreover, the optical fiber used to convey the collected resultant light can couple to small sensor area effectively. In some examples, the NA of the collection optics is smaller than the NA of the fiber. In some examples, a long pass filter is positioned between NF and fiber. In some examples, a filter can be used as discussed herein with reference to FIG. 24 . In some examples using a long pass filter to reduce laser leakage, only single-photon pulses are measured. In some examples, an OD6 filter is not used.

Experiments were conducted to measure cps as a function of time for various tested examples. Tests were performed of Quartz/Glass photo bleaching at a high level of exposure, as indicated by the total photon count. The tests were conducted with a 405 nm laser emitting 500 μW and a 100 μm-diameter (ϕ) beam waist. Three tests were conducted, respectively with: a quartz 0.1 mm thick, cover glass 0.17 mm thick, and cover glass 0.1 mm thick. The plotted exposure data were collected over the course of ten minutes (600 s). Quartz exhibited the lowest total count and intermediate photo bleaching, 0.1 mm cover glass exhibited the highest total count and high photo bleaching, and 0.17 mm cover glass exhibited an intermediate total count and low photo bleaching (relatively little change in output over the course of the ten minutes).

FIG. 19 shows some components and optical paths of an example motorized monochromator. The illustrated monochromator can be configured to receive broadband illumination via the entrance slit and provide illumination of a narrow bandwidth (e.g., only a few nm) via the exit slit. The motor can turn or otherwise adjust the grating or other optical components of the monochromator to select which wavelengths are collected and counted.

There are various sources of stray light in a monochromator. Some include stray light from the entrance, stray light from the exit, and AFL from the inside walls of the monochromator. Other sources include 2nd order and 0th order light from the grating, in configurations in which the 1st order light is normally used. To reduce stray light and improve detection sensitivity when using monochromators and other optical enclosures, various examples use a metamaterial absorber. For example, carbon nanotubes (CNTs) with or arranged in metastructure can absorb photons to a level of less than 0.1% reflectivity, compared to a typical anodized Al coating reflectivity of about 4%. Improvements in reflectivity can directly improve dynamic range. For example, a reduction in reflectivity to 1% of its former levels can improve the dynamic range of a spectrometer by 100 times.

Measured data was collected at various wavelengths for a tested monochromator. Calculated addresses for the wavelengths were compared with the measured data. The tested monochromator agreed with the calculated results.

In some examples, systems such as that shown in FIG. 1 can provide photon counts for each of a plurality of wavelength intervals at each of a plurality of times (or in each of a plurality of time windows). For example, the wavelength windows can be 0.1-10 nm, the scan times for a particular wavelength can be 1-1000 s, and the exposures can be captured over 1 ms-10.00 s. However, other values outside these ranges can additionally or alternatively be used. In some examples, 1 s samples over a long period of time, e.g., >600 s, can be used to measure photo bleaching. In some examples, the dark count can be about 125 cps.

An experiment was conducted to measure data of a single-photon spectrum of a sheath fluid for a flow cytometer, as measured by a system such as that shown in FIG. 1 including a motorized monochromator. The system background count was measured after installing, successively, a 440 nm long-pass filter, a tube holder, and dry tubing. Measurements were then taken after liquid sample filling to determine autofluorescence (AFL) of a tested sheath liquid for a flow cytometer, e.g., as discussed herein with reference to FIG. 24 . The system was able to effectively measure the sheath liquid, with cps peaking over 450,000 compared to a background level after installing the dry tubing of less than 50,000.

FIG. 20 shows an example configuration of an optical system permitting wall-less fluorescence detection of liquid samples by coaxial illumination in a tube. In some examples of wall-less detection, no vessel is used in a manner that would cause the vessel to exhibit autofluorescence. Abbreviations are as discussed herein with reference to FIGS. 17 and 18 .

Many materials exhibit autofluorescence (AFL), including high-grade quartzes and glasses. In some prior schemes, liquid samples require a vial for measurement. However, measurements of the sample fluorescence can be affected by AFL of the vial walls, i.e., photons emitted by the vial walls because of AFL. Therefore, wall-less capturing of sample liquid permits fluorescence detection with reduced noise from vessel AFL. In some examples, the sample liquid is held by surface tension within a 2-3 mm (inner diameter) glass tube, e.g., having relatively flat inner surfaces and open ends. This size is nonlimiting; other sizes can be used as long as the sample is retained within the tube by surface tension. Other materials than glass can be used for the tube, e.g., quartz. The sample is illuminated along a coaxial direction, e.g., substantially parallel to the walls of the tube. This can significantly reduce the amount of incident light reaching the tube walls, so can substantially reduce measurement noise due to vial wall auto fluorescence. In some examples, 10 μL-20 μL of sample fluid is used to provide a 2 mm sample thickness. In some examples, e.g., as discussed herein with reference to FIGS. 30 and 31 , the tube can be a tube or well in an array of tubes, e.g., a plate array. Plates can include, e.g., 96 tubes, 384 tubes, or more tubes. In some examples, the tube has an open end and retains a fluidic sample (e.g., a liquid sample) by capillary action.

In some examples, the tube, the laser source, or other components of a measurement system are arranged so that the beam does not cross or intersect with the tube within a light-collection area associated with the tube and the sample. In the illustrated example, the light-collection area is a portion of the sample or its surroundings from which resultant light is directed to the collection fiber by the collection lens or other optics. If the beam crosses the tube away from the light-collection area, AFL photons will not be directed to the collection fiber, so will not corrupt the measurement. In some examples, the light-collection area comprises at least the smallest cylindrical volume that is coaxial with the incident light, that extends longitudinally only far enough to encompasses the entirety of the sample, and that extends radially only as far as necessary to encompass (or, in some examples, reach) the outer walls of the tube.

In some examples, measurement systems as described herein can be used for automated drug screening or other high-volume biological measurement tasks. For example, in automated high throughput screening, multiple samples can be arranged in separate tubes along the path of a laser. The fluorescence of each sample can be measured individually, permitting measuring more samples in a given amount of time. The dynamic range and wavelength selectivity can permit performing advanced chemical analyses, such as drug screening, not supported by some prior schemes.

FIGS. 21A-21G show examples of aperture shape conversion fiber bundles configured to provide increased spectral resolution and coupling efficiency. In some examples, a bundle of fibers is used to convey light between the monochromator slits and other optical components.

FIG. 21A shows spectrum 2100, e.g., as spatially distributed by a grating such as that shown in FIG. 19 . Overlaid on spectrum 2100 are example fiber-bundle shapes 2102 (circular) and 2104 (rectilinear, short axis along the spectrum).

FIG. 21B shows an example monochromator for selecting light of a particular wavelength from a broadband source such as sunlight.

FIG. 21C is a cross-sectional view of an example of dense packing of circular optical fibers.

FIGS. 21D and 21E show cross-sections of fibers bundled into circular bundles 2102. Such configurations can be used, e.g., at the interfaces to the laser, lenses, sensors, or sample.

FIGS. 21F and 21G show cross-sections of fibers bundled into substantially rectilinear configurations 2104. Such configurations can be used at the monochromator entrance and exit slits to improve spectral resolution.

In the illustrated example, bundles A and B (FIGS. 21D and 21E) are circular bundles, so have aspect ratios (long side:short side) of 1:1. Bundles C and D (FIGS. 21F and 21G) are rectangular bundles. Bundles C and D are examples of bundles having an elongated shape, e.g., bundles for which the best-fit ellipsoid is substantially non-circular. For example, for bundles C and D, the elongated shape is a substantially rectangular shape. For bundle D, the elongated (rectangular) shape has an aspect ratio (long:short) exceeding 3:1, specifically, 700:186 3.8. In some examples, the elongated shapes have aspect ratios of at least, or greater than, 2:1. Fiber bundles can provide improved power efficiency while maintaining spectral selectivity compared to some prior schemes.

Some example monochromators have performance figures of merit expressed in spectral width vs. slit width, and exit intensity vs. slit width. In some examples, narrower slit widths of the monochromator generally correspond with narrower wavelength bands into or out of the monochromator, which can increase spectral resolution. For example, a wider slit has larger signal but less spectral resolution compared to a narrower slit. In some examples, a conversion fiber bundle converts from circular close packing to linear packing, e.g., as in FIG. 21 . This can provide increased signal and increased spectral resolution.

FIGS. 21F and 21G show examples of proximal ends of bundles. A bundle can receive, at its proximal end, at least one of a spatially dispersed plurality of wavelengths, e.g., from a monochromator exit slit or while scanning across the output field of a polychromator (e.g., FIG. 26B). The distal end of the bundle can be arranged to provide light to an SPM, e.g., as in FIG. 27B.

An experiment was performed to measure data of a test of conjugate excitation in Quartz 7980, and the effect of conjugated optics. The measurement equipment used for this test included a single core fiber. The sensor's dark count level was measured from 350-850 nm, as was the system's background count curve (higher than the dark count level across the range, but less so at longer wavelengths). Results were measured across that range of one-pass excitation of the quartz material, and showed a peak around 40,000 cps near 510 nm, compared to a background level of about 19,000 cps. A spectral profile was then measured by conjugated excitation, in which the quartz material receives double the illumination due to reflection of the incident light off the notch filter after passing through the quartz material the first time. The ˜510 nm peak of the two-pass condition was ˜71,000 cps, over twice the counts per second of the one-pass.

An experiment was performed to measure the spectral profile, after subtracting the background, of conjugate excitation in Quartz 7980. An intensity profile was measured by conjugated excitation. A separate intensity profile was measured by one-pass excitation. The double-excitation curve had a peak 2.57× higher than the peak of the single-excitation curve (51.7 kcps vs. 20.1 kcps).

An experiment was performed to measure data of cps by wavelength (350-510 nm) for, e.g., various diameters of fiber core. In the tested condition, using a core diameter >0.6 mm, the spectral FWHM was larger than the sampling resolution. The FWHM as a function of core diameter was as shown in Table 2.

TABLE 2 Slit width FWHM (nm) 0.2 mm 1.5 0.3 mm 2.0 0.4 mm 2.8 0.5 mm 3.4 0.6 mm 4.0 0.7 mm 4.6 0.8 mm 5.0

FIG. 22 shows an end view of an example using a relatively large-core fiber and a slit aperture at the end of the fiber. This combination can provide improved coupling efficiency and spectral resolution compared to various prior schemes. The slit aperture can provide increased selectivity with reduced complexity compared to fiber bundles.

An experiment was performed to measure data of optical efficiency (in percent) and of FWHM (nm) as a function of slit width in mm. As slit width increased, optical efficiency and FWHM both increased. Below a slit width of 0.6 mm, efficiency dropped more rapidly as slit width decreased than was the case above 0.6 mm. The experiment showed that FWHM was controlled by the input core size.

FIG. 23 shows components of an example optical system supporting on-axis and perpendicular measurement, in any combination. The illustrated components can additionally or alternatively be applied at other angles than those shown, e.g., 45°. In some examples, the illustrated sample can be located in a flow chamber, e.g., as discussed herein with reference to FIG. 24 . In some examples, detectors using a monochromator as described herein (e.g., FIG. 17, 19, 21B, or 26A), or a polychromator as described herein with reference to FIG. 26B, can be used to measure the on-axis and perpendicular resultant light. Some examples can use a slit and large core combination (FIG. 22 ) with perpendicular detection. The on-axis and perpendicular configurations are shown for purposes of example; single photon spectroscopy as described herein can be applied at any angle with respect to the incident beam. For example, on-axis detection can be used with transparent samples and conjugate excitation. Perpendicular detection can be performed without a filter, so can have reduced spectral loss.

The Influence of excited AFL depends on optics configuration. In-line detection to laser incident direction may be more affected than perpendicular fluorescence detection. In the case of confocal fluorescence imaging or flow cytometry with longer gate period for small particles, optics AFL can be reduced for higher contrast and accurate detection.

An experiment was performed to measure data of photon spectra of two different quartz samples, in cps over the wavelength range (in nm). In the tested example, the two types of quartz were distinguishable from each other using single-photon measurements.

The experimental data were collected in a step mode. In some examples, counting can be performed in, e.g., a step mode or a scan mode. In step mode, the monochromator can be adjusted to a particular wavelength (e.g., a 1 nm window), photons can be collected for a predetermined time (e.g., 1 s) while the monochromator is substantially fixed with respect to wavelength, and the adjusting and collecting operations can be repeated to collect respective photon counts for each of a plurality of wavelengths.

In scan mode, by contrast, the monochromator can be adjusted to change the wavelength continuously at a predetermined rate. For example, the monochromator can scan a 2 nm-wide window across the spectrum, and data can be collected, e.g., substantially continuously, or at points or intervals in time associated with predetermined wavelengths (e.g., the window centered on 400 nm, 405 nm, 410 nm, . . . ).

FIG. 24 is a perspective view schematically showing an example flow-cytometry system 3800 that can be used with sensors and detection systems described herein, e.g., FIG. 23 . System 3800 includes a flow cell 3802. The flow cell 3802 includes a flow chamber 3804, and is, at least in part, transparent or substantially transparent to irradiation such as light L and resultant light such as light L_T-FS and L_T-SS. For clarity, only part of flow chamber 3804 is shown. Further details of various configurations of flow chamber 3804 and other parts of a flow system 330 (FIG. 3 ) are discussed below with reference to FIGS. 3A, 3B, and 5-10B. As shown, the flow cell 3802 can be 2 mm thick along the direction of propagation of light L.

A sheath flow SH flows into the flow cell 3802 from an inlet port IN1. For example, saline, which is an isotonic liquid, or water, can be used as the sheath flow SH. However, the sheath flow SH is not limited to saline, but various types of liquid such as water, other aqueous solutions (whether isotonic or not), and organic solvents can be used. In various examples, the sheath flow SH also flows into the flow cell 3802 from at least one additional inlet port, shown as IN3.

Further, a sample flow SM including microparticulate samples or other targets to be analyzed flows into the flow cell 3802 from an inlet port IN2. For example, saline, which is an isotonic liquid, can be used as the sample flow SM. However, the sample flow SM is not limited to saline, but various types of liquid such as water, other aqueous solutions (whether isotonic or not), and organic solvents can be used. The inflow pressure of the sample flow SM can be higher than or lower than the inflow pressure of the sheath flow SH. Flow chamber 3804 or other fluid channels in flow cell 3802 can be arranged so that the center of the sample flow is the fastest and the flow velocity approaches zero at the walls of the flow channel 3804. This can cause targets to be hydrodynamically focused, i.e., positioned by the fluid flow, substantially in the center of the sample flow. In the illustrated example, the fluid flows SM and SH are provided by a fluidic supply 3806.

The inlet ports IN1, IN2, IN3 can be bored, molded, or otherwise formed in the flow cell 3802. In an example, the flow cell 3802 includes glass or quartz. For example, flow channels (e.g., flow chamber 3804) can be formed by micro-blasting of quartz sheets. Ports IN1, IN2, IN3 can be drilled out of the quartz sheets. Other etching and boring techniques can be used to form flow channels, inlets, and other features. For example, sample channels, including flow chamber 3804, can be etched, and sheath channels can be micro-blasted using a mask to define the desired pattern. In other examples, channels and other cavities described herein can be injection molded, molded using other techniques, bored, or etched.

The sheath flow SH and the sample flow SM merge in the flow chamber 3804, so that a flow FL is provided in which the sample flow SM is substantially hydrodynamically focused with the sheath flow SH, e.g., around the sample flow SM, or arranged in other hydrodynamic-focusing configurations. The flow FL can be discharged to the outside of the flow cell 3802 in some examples.

An optical source 3808, e.g., a laser or other illumination source, can provide light L aimed, focused, or otherwise directed to irradiate the targets entrained within the sample flow SM. Laser light L can be at least partly transmitted or at least partly scattered, providing resultant forward-scattered light L_T-FS and resultant side-scattered light included in L_F-SS. Targets, e.g., chromophores bound to target molecules of interest, can fluoresce, producing resultant fluorescent light also included in L_F-SS. A detector 3810, e.g., an on-axis detector (FIG. 23 ), can detect light L_T-FS. A detector 3812, e.g., a perpendicular detector (FIG. 23 ), can detect light L_F-SS. Various examples can use one detector or more than one detector. Detectors can be placed at any angle with respect to the axis of the light L.

In some examples, optical source 3808 can include at least one component described in at least one of FIG. 1, 16, 17, 18, 19, 20, 21, 22, 23 , or 25. In some examples, at least one of detector 3810 or detector 3812 can include at least one component described in at least one of FIG. 1-5, 7, 9-11C, 13, 14, 17-23, 25, 28 , or 32-37. For example, optical source 3808 can include a laser and conjugate focusing optics, and detector 3810 can include a motorized monochromator, a SiPM using Geiger-mode PDs, and a differential detection circuit (FIGS. 4A-6B).

In some examples, a controller (“CTL”) 3814 can control operation of the fluidic supply 3806 or the optical source 3808. In some examples, controller 3814 can receive information, e.g., photon counts, from detectors 3810, 3812.

Example Features

Various examples relate to multiple photon detection, e.g., with <1 ns pulse width, e.g., by avalanche signal processing. Various examples relate to spectrum analysis. Various examples relate to or include at least one of the following features, in any combination:

-   -   1. High pass filter Signal Processing: E.g., C_(HP)         (Farad)=T_(RISE)/50, e.g., t=100 ps, c=2 pF. E.g., C_(HP)         (Farad)=T_(RISE)/25˜100     -   2. Adaptive pedestal clamping by ADD signal processing—Add two         signals using a gain that stabilizes the pedestal level. In some         examples, the two signal phase difference is smaller than 10         pulse pairs in resolution.     -   3. Differential Signal Processing of Photon Pulse: e.g.,         C_(DIFF) (Farad)=(⅕)C_(HP)=T_(RISE)/250. In some examples, the         primary corner frequency F0 (Hz)=1/(10 T_(RISE))     -   #3, using Differential Amplitude: C_(DIFF) (Farad)=(⅕)         C_(HP)=T_(RISE)/250; Primary corner frequency F0 (Hz)=1/(10         T_(RISE))     -   #3, using Differential Pulse Width: Two comparators at a         threshold level of photon pulses; pulse width discrimination by         pulse width comparison.     -   4. Adaptive comparator: Adaptive Envelope reference level         determined as constant+envelope signal.     -   5. Comparator pulse resolution: smallest resolvable pulse         smaller than pulse width at 70% level of Pulse height.     -   6. Photon Spectrum Analyzer including at least one of the         following: Excitation Laser; focus lens; bandpass filter (laser         wavelength); sample illumination; collection lens; notch filter;         fiber-motorized monochromator; fiber-photon detection.     -   #6, including a scanning monochromator for synchronized spectrum         window Δλ at x seconds.     -   #6, including photon counting per each Δλ, and repeated         exposures.     -   #6, configured to provide data indicating n×Δλ vs. photon count         as a histogram.     -   #6, configured to provide data indicating a counting trend at a         specific wavelength (photon energy).     -   7. Spectrometer including a coherent-light illumination unit, a         monochromator, and a single photon detector.     -   #7, where the optics includes illumination optics and collection         optics that convey the light to an optical fiber.     -   #7, where the optics are arranged in an in-line layout between         illumination and detection.     -   #7, where the illumination optics focus the laser beam on a         reflective laser notch filter. This way, the light excites the         sample twice: once before reflection and once after. The sample         is located close to the laser beam waist and the notch filter.     -   #7 or the preceding, where the Illumination optic include a         laser band pass filter to reduce laser spontaneous emission         (e.g., ASE).     -   #7, where the collection optics collects fluorescence photon in         specific solid angle defined by a NA.     -   #7, where the monochromator IN and OUT fiber bundles have slit         shapes with close packing at the monochromator.

The preceding, where the fiber bundles have circular shapes away from the monochromator.

-   -   8. Tube Trapping of a liquid sample. This can reduce vial AFL.     -   9. Fluorescence photons scanned by a motorized monochromator for         spectral analysis.     -   10. Data is analyzed for wavelength and time dependence, e.g.,         photo-bleaching.     -   11. Estimating and analyzing fluorescence characteristics         quantitatively based at least in part on photon count, photon         energy, illumination power, optics NA, or sensor PDE.     -   12. Any of the foregoing example features, combined or used with         components described herein with reference to FIG. 2A-2D, 4A-4C,         or 32-34.     -   13. Examples described herein with reference to FIG. 2A-2D,         4A-4C, or 32-34.

Various aspects described herein provide detection and quantitative measurement (e.g., counts) of photons, e.g., using an SiPM or SSPMT. A technical effect is to measure emissions of fluorescent targets, e.g., in a sample fluid. A further technical effect is to effectively detect photons that strike a SiPM during the quench/recharge/dead time of that SiPM.

In some examples, SiPM or SSPMT Photon Detectors as described herein can provide improved sensitivity and linearity of fluorescence analysis. A dynamic range of six orders of magnitude can be measured, compared to three orders of magnitude for conventional photocurrent analysis. In some examples, flow cytometry systems herein can apply high power laser beam illumination to a sample in order to analyze material characteristics at high intensity illumination. Some examples can detect photo-bleaching phenomena using single photon analysis, unlike some prior schemes. Some examples can reduce auto fluorescence compared to some prior cytometers, e.g., by removing AFL effects due to a quartz flow chamber, sheath fluid, water, or calibration microbeads. Some examples permit cellular analysis by determining material AFL and providing very-low-fluorescence detection, e.g., of microparticles or single molecules.

Example Data-Processing System

FIG. 25 is a high-level diagram 3900 showing the components of an example data-processing system 3901 for analyzing data and performing other analyses described herein, and related components. The system 3901 includes a processor 3986, a peripheral system 3920, a user interface system 3930, and a data storage system 3940. The peripheral system 3920, the user interface system 3930, and the data storage system 3940 are communicatively connected to the processor 3986. Processor 3986 can be communicatively connected to network 3950 (shown in phantom), e.g., the Internet or a leased line, as discussed below. Devices above (e.g., the PC in FIG. 1 , the system in FIG. 9 , the PC in FIG. 17 , controller 3814 shown in FIG. 24 , the PC shown in FIG. 34 , or other processing systems herein) can each be or include one or more of systems 3901, 3986, 3920, 3930, or 3940, and can each connect to one or more network(s) 3950. Processor 3986, and other processing devices described herein, can each include one or more microprocessors, microcontrollers, field-programmable gate arrays (FPGAs), application-specific integrated circuits (ASICs), programmable logic devices (PLDs), programmable logic arrays (PLAs), programmable array logic devices (PALs), or digital signal processors (DSPs).

Processor 3986 can implement processes of various aspects described herein. Processor 3986 and related components can, e.g., carry out processes for detecting photons, collecting count data from counters, operating a laser (e.g., adjusting the laser power), operating a monochromator (e.g., to scan across a wavelength band), or operating a fluid supply or other components of a flow-cytometry system as in FIG. 24 .

Processor 3986 can be or include one or more device(s) for automatically operating on data, e.g., a central processing unit (CPU), microcontroller (MCU), desktop computer, laptop computer, mainframe computer, personal digital assistant, digital camera, cellular phone, smartphone, or any other device for processing data, managing data, or handling data, whether implemented with electrical, magnetic, optical, biological components, or otherwise.

The phrase “communicatively connected” includes any type of connection, wired or wireless, for communicating data between devices or processors. These devices or processors can be located in physical proximity or not. For example, subsystems such as peripheral system 3920, user interface system 3930, and data storage system 3940 are shown separately from the processor 3986 but can be stored completely or partially within the processor 3986.

The peripheral system 3920 can include or be communicatively connected with one or more devices configured or otherwise adapted to provide digital content records to the processor 3986 or to take action in response to processor 186. For example, the peripheral system 3920 can include digital still cameras, digital video cameras, cellular phones, or other data processors. The processor 3986, upon receipt of digital content records from a device in the peripheral system 3920, can store such digital content records in the data storage system 3940. In the illustrated example, the peripheral system 3920 permits the processor 3986 to control fluidic supply 3806 and optical source 3808. The peripheral system 3920 also permits the processor 3986 to receive data from detector(s) 3902, e.g., detectors 3810 or 3812. In some examples, the peripheral system 3920 also permits the processor 3986 to control a spectral discriminator 3960, e.g., a motor or other drive that operates a monochromator to select the particular wavelength band output by the monochromator (e.g., FIG. 19 or 26 ).

The user interface system 3930 can convey information in either direction, or in both directions, between a user 3938 and the processor 3986 or other components of system 3901. The user interface system 3930 can include a mouse, a keyboard, another computer (connected, e.g., via a network or a null-modem cable), or any device or combination of devices from which data is input to the processor 3986. The user interface system 3930 also can include a display device, a processor-accessible memory, or any device or combination of devices to which data is output by the processor 3986. The user interface system 3930 and the data storage system 3940 can share a processor-accessible memory.

In various aspects, processor 3986 includes or is connected to communication interface 3915 that is coupled via network link 3916 (shown in phantom) to network 3950. For example, communication interface 3915 can include an integrated services digital network (ISDN) terminal adapter or a modem to communicate data via a telephone line; a network interface to communicate data via a local-area network (LAN), e.g., an Ethernet LAN, or wide-area network (WAN); or a radio to communicate data via a wireless link, e.g., WIFI or GSM. Communication interface 3915 sends and receives electrical, electromagnetic, or optical signals that carry digital or analog data streams representing various types of information across network link 3916 to network 3950. Network link 3916 can be connected to network 3950 via a switch, gateway, hub, router, or other networking device.

In various aspects, system 3901 can communicate, e.g., via network 3950, with a data processing system 3904, which can include the same types of components as system 3901 but is not required to be identical thereto. Systems 3901, 3904 can be communicatively connected via the network 3950. Each system 3901, 3904 can execute computer program instructions to operate measurement systems or capture measurements as described herein, or to communicate measurement data, e.g., via network 3950.

Processor 3986 can send messages and receive data, including program code, through network 3950, network link 3916, and communication interface 3915. For example, a server can store requested code for an application program (e.g., a JAVA applet) on a tangible non-volatile computer-readable storage medium to which it is connected. The server can retrieve the code from the medium and transmit it through network 3950 to communication interface 3915. The received code can be executed by processor 3986 as it is received, or stored in data storage system 3940 for later execution.

Data storage system 3940 can include or be communicatively connected with one or more processor-accessible memories configured or otherwise adapted to store information. The memories can be, e.g., within a chassis or as parts of a distributed system. The phrase “processor-accessible memory” is intended to include any data storage device to or from which processor 3986 can transfer data (using appropriate components of peripheral system 3920), whether volatile or nonvolatile; removable or fixed; electronic, magnetic, optical, chemical, mechanical, or otherwise. Example processor-accessible memories include but are not limited to: registers, floppy disks, hard disks, tapes, bar codes, Compact Discs, DVDs, read-only memories (ROM), erasable programmable read-only memories (EPROM, EEPROM, or Flash), and random-access memories (RAMs). One of the processor-accessible memories in the data storage system 3940 can be a tangible non-transitory computer-readable storage medium, i.e., a non-transitory device or article of manufacture that participates in storing instructions that can be provided to processor 3986 for execution.

In an example, data storage system 3940 includes code memory 3941, e.g., a RAM, and disk 3943, e.g., a tangible computer-readable rotational storage device or medium such as a hard drive or solid-state drive (SSD). Computer program instructions are read into code memory 3941 from disk 3943. Processor 3986 then executes one or more sequences of the computer program instructions loaded into code memory 3941, as a result performing process steps described herein. In this way, processor 3986 carries out a computer implemented process. For example, steps of methods described herein, blocks of the flowchart illustrations or block diagrams herein, and combinations of those, can be implemented by computer program instructions. Code memory 3941 can also store data, or can store only code.

Furthermore, various aspects herein may be embodied as computer program products including computer readable program code (“program code”) stored on a computer readable medium, e.g., a tangible non-transitory computer storage medium or a communication medium. A computer storage medium can include tangible storage units such as volatile memory, nonvolatile memory, or other persistent or auxiliary computer storage media, removable and non-removable computer storage media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. A computer storage medium can be manufactured as is conventional for such articles, e.g., by pressing a CD-ROM or electronically writing data into a Flash memory. In contrast to computer storage media, communication media may embody computer-readable instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transmission mechanism. As defined herein, computer storage media do not include communication media. That is, computer storage media do not include communications media consisting solely of a modulated data signal, a carrier wave, or a propagated signal, per se.

The program code includes computer program instructions that can be loaded into processor 3986 (and possibly also other processors), and that, when loaded into processor 3986, cause functions, acts, or operational steps of various aspects herein to be performed by processor 3986 (or other processor). Computer program code for carrying out operations for various aspects described herein may be written in any combination of one or more programming language(s), and can be loaded from disk 3943 into code memory 3941 for execution. The program code may execute, e.g., entirely on processor 3986, partly on processor 3986 and partly on a remote computer connected to network 3950, or entirely on the remote computer.

Further Example Embodiments, Features, and Experimental Results

An experiment was performed to measure PE characteristics, in cps by mV, at bias voltages from 27 V-29 V, by 0.5 V. As bias voltage increased, a given mV level produced higher CPS. The five tested curves showed generally the same shape (a double waterfall), but shifted to the right (+mV) and slightly up (+cps) as bias voltage increased.

An experiment was performed to measure photo bleaching, indicated as the cps of the spectral peak as a function of accumulated exposure in mJ. The observed photo-bleaching occurs over a period of time, as the exposure accumulates. In the tested example, cps peaked at 23 kcps at <10 mJ of exposure, and declined to 2.6 kcps after about 280 mJ of exposure.

An experiment was performed to determine the amount or extent of photo bleaching using normal and reverse scans. In the illustrated example, measurements were collected from short wavelengths to long wavelengths (normal scan), 420 nm-670 nm. Subsequently, measurements were collected from long wavelengths to short (reverse scan). The difference between the two measurements at a given wavelength indicates the photo-bleaching at that wavelength. In some examples, the reverse scan can be performed before the normal scan. In the tested example, the normal scan peaked at (446 nm, 110 kcps). The corresponding peak on the reverse scan was at around 55 kcps. The curve shapes were generally the same.

An experiment was performed to measure spectra of a multi-ion discharge lamp in cps as a function of wavelength from 400-700 nm using techniques described herein. The measurements showed strong peaks at ˜430 nm and ˜540 nm. Techniques described herein can additionally or alternatively be used to measure other types of lamps, e.g., to test conformance of standard lamps or other sources to corresponding illuminants.

The experimental data covered more than three orders of magnitude (from below 103 cps to above 10⁶ cps). Accordingly, some prior schemes having at most three orders of magnitude of dynamic range would not be able to effectively measure the lamp to provide the experimental data. Moreover, some prior lamp-characterization systems provide results in arbitrary units. As discussed above, counts measured by some example detectors described herein can be readily converted to physical units. This can make the collected data applicable to a wider range of uses than data provided by some prior schemes.

An experiment was performed to measure spectra using techniques described herein, in cps as a function of wavelength. A tungsten-halogen light source was measured without a filter, and found to have broadband emission over the range 400-700 nm. Two filters were measured: “525BPA” and “525BPB”. The wide dynamic range (cps as high as ˜1.6×10⁷) and possibility to report results in physical units permit characterizing filters more accurately than in some prior schemes. This can permit improving the quality of data from flow cytometry, since filter characteristics can significantly affect cytometric results.

An experiment was performed to measure spectra (log scale) of various colors of physical test targets: Semi-transparent sky blue; Largely opaque violet; Semi-transparent red; Semi-transparent pink; Semi-transparent orange; Semi-transparent yellow; Semi-transparent green; Semi-transparent blue.

The physical test targets were plastic POST-IT flags of different colors. The measurements were able to span more than four orders of magnitude, from a system baseline of ˜104 cps to a peak measurement ˜1.2×10⁸ cps. This is much more dynamic range than some prior schemes.

FIG. 26A shows a perspective of an example motorized monochromator. Example monochromators are described herein with reference to FIGS. 17, 19, and 21 .

FIG. 26B shows an elevational cross-section of an example polychromator. The polychromator can, e.g., disperse incoming light across to a fiber-optic cable bundle, e.g., as in FIG. 21 or 22 . Additionally or alternatively, the polychromator can include or be connected to a motion system (e.g., a linear stage) that translates a fiber or fiber bundle across the spectrum spatially distributed by the grating.

FIG. 27A shows an example 42-channel (“42CH”) linear fiber array connector 2700. The illustrated connector includes a bundle 2702 of 42 optical fibers arranged along a line.

FIG. 27B shows output fibers fed by a polychromator such as that shown in FIG. 26 . Connector 2700 can connect to the polychromator and carry the output fibers in bundle 2702. As noted by the color labels, different output fibers are carrying different wavelengths of output light.

FIG. 28 shows an example SiPM linear array configured for high-speed (e.g., μs) single-photon spectrum detection. The array can be used as a photon sensor or optical detector in various configurations herein, e.g., FIG. 1, 9, 17, 18, 20, 24 , or 25. As shown, the array includes multiple blocks of SiPM pixels (e.g., 250 pixels each, as 5 px×50 px). Each SiPM pixel can include a PD and a resistor, e.g., as discussed herein with reference to FIG. 2A, 3A, 3B, 4A, or 11A-11C. Each block of pixels extends substantially along an axis that is substantially not parallel with (e.g., is substantially perpendicular to, or is at least 40° from) a direction (horizontal) across which the spectrum is spread. Multiple blocks are arranged along the spectrum-spread direction (e.g., 256 blocks for a 256-channel array). Any number of blocks, or of pixels per block, can be used. In the illustrated example, the array is or includes a monolithic SiPM array. In some examples, electronics, e.g., as in FIG. 4A, 5, 7, 9, 10A-10B, 13 , or 14, can be integrated within a die or package holding the SiPM array. In some examples, multiple array or electronics dice can be integrated within a package. The illustrated array has 256 blocks (shown as strips extending vertically), each corresponding to one output channel. The blocks are packed together horizontally. Each illustrated block can have a width of 100 μm, for a total array width of ˜25.6 mm Each block can be, e.g., 1 mm long. Each block can include 250 pixels, e.g., arranged in five rows of 50 pixels each (as shown in the inset). In some examples, the blocks can be separate blocks, e.g., spaced apart or abutted. In some examples, the blocks can be portions of an area sensor having a regular 2-D array of SiPM pixels. In some examples, the linear array can comprise an area sensor.

In some examples, the dark count of a SiPM sensor is proportional to sensor area. Therefore, using small blocks (e.g., 1.0×0.1 mm as shown) can reduce the dark current (e.g., to one-tenth that of a 1×1 mm sensor). Cooling, filtering, or other techniques herein can further reduce the dark count.

Dark-count rate (DCR) is the lower limit of sensitivity and largely determines the dynamic range of photon counting. A cause of high dark counts is thermal noise in p-n junctions. Thermal electrons are amplified in a similar manner to incident photons. Unlike photo current noise (thermal noise, shot noise, amp noise, and so on), the dark count can easily be subtracted from the evaluated count rate. A Peltier cooler or other cooling unit can be arranged proximal to the SiPMs or other SSPDs and configured to cool the SSPDs to reduce dark count. Other examples of coolers include forced-air ventilation coolers, e.g., cooling heatsinks applied to the SiPMs; liquid-cooling systems; or cooling systems using dry ice or other compounds with freezing temperatures below 0° C., either in direct contact with the SSPDs to be cooled or arranged so that the vapor from those compounds is brought into thermal contact with the SSPDs to be cooled.

A 1 mm² SiPM sensor was tested. At temperatures higher than 25° C., the DCR was over 100 kcps, but was reduced to 2 kcps at −10° C. Various examples provide a DCR <1 kcps by cooling the sensor below −20° C. Experimental evidence showed that a DCR of 100 cps was attained by dry-ice cooling at −50° C. A counting range from 1 kcps to 1 Gcps provides a six-orders-of-magnitude dynamic range with theoretical linearity in the digital environment. Temperature control also provides improved data accuracy, stability, and reproducibility. The measured value of DCR at 4° C. was 5 kcps, the standard deviation per second, r, was 10 to 50 cps, and the coefficient of variation was 0.2-1.0%, respectively.

FIG. 29 shows an isometric view of an example optical system and array. Resultant light is shown for clarity of explanation as a collimated beam, although this is not required. The illustrated optics collect resultant light and spread the light over the array (five blocks are shown, but any number can be used). The optics causes different wavelength bands of light to strike each block (“H spreading”). For a particular block, the optics can spread the light within the corresponding wavelength band across the block to increase sensitivity or dynamic range of the detection system (“V spreading”). In some examples, H and V spreading can be performed by separate optics, e.g., a beam expander for V spreading followed by a grating for H spreading. In some examples, only H spreading is performed (e.g., for beams already substantially as wide as the blocks are long). V spreading can additionally or alternatively be performed by directing the beam onto the sensor at an angle other than normal to the sensor, e.g., from below or above in the illustrated configuration, to expand the width of the beam as projected on the sensor.

Accordingly, the optics can be configured to spread out different wavelengths of the resultant light along the same direction the blocks are spread out (horizontal, as illustrated) and to spread out light within a particular wavelength along a second direction substantially different from that direction (vertical, as illustrated), e.g., a direction perpendicular to or more than 40° from that direction. The illustrated optics can include at least one slit, grating, beam shaper, beam expander, prism, mirror, or lens, in any combination. In an example, optics includes a spectral discriminator, e.g., a grating or prism, to perform the H spreading, followed by a slit or a lens arrangement to perform the V spreading. The illustrated orientation, and the terms “H” and “V,” are not limiting; other orientations or spatial arrangements can be used.

In some examples, the optics can include a toroidal mirror grating for the H spreading. Additionally or alternatively, the optics can include a plurality of dichroic mirrors configured to reflect respective wavelengths to respective blocks or groups of blocks. Additionally or alternatively, the optics can include prism(s).

FIG. 30 shows an example system for high-throughput sample measurement or analysis. Insets are indicated using dash-dot lines. A plate array, e.g., a substantially transparent microwell plate, can hold a plurality of samples, e.g., fluidic samples. In some examples, e.g., as discussed herein with reference to FIG. 20 , the plate can include open tubes, and samples can be held in the tubes by capillary action. A laser beam or other incident light can be directed into each tube, e.g., successively in a raster scan across the plate. Additionally or alternatively, multiple wells can be illuminated at once with respective laser beams or other incident light, e.g., from multiple light sources or from one source split, e.g., using partially-silvered mirrors, diffraction gratings, or other optical elements, into multiple beams.

Resultant light from the sample, e.g., fluorescent light or other light as in FIG. 1, 17, 18, 20, 23 , or 24, can travel out of the indicated tube (e.g., microwell), e.g., substantially in the direction of travel of the laser or other incident light. This can reduce crosstalk between wells. Other directions of resultant-light travel can additionally or alternatively be used. A detector, e.g., as in FIG. 1, 17, 18, 20, 21, 23, 24, 25, 28 , or 29, can detect the resultant light. One tube at a time can be measured, or multiple tubes can be measured concurrently using multiple detectors.

In the illustrated example, the system has R rows of wells and C columns of wells. In some examples, the system is configured with a detector for each row, or for each column, and an irradiation system configured to provide incident light to each tube in a column, or each tube in a row, respectively. This can permit measuring an entire row or an entire column at once, which can increase throughput.

FIG. 31 is an isometric view of tubes in a portion of a plate array such as that in FIG. 30 . Illustrated are six tubes and, for each, a respective path of the laser beam or other incident light.

Still Further Example Embodiments and Features

FIG. 32 shows a plan view of an example solid-state silicon microelectromechanical system (MEMS) photomultiplier tube (PMT), e.g., sold by HAMAMATSU. The illustrated SSPMT is an example of an SSPD, and can be used instead of or in addition to SiPMs in measurement systems described herein. Unlike a conventional PMT with metal dynode structure, the micro-PMT dynode is made by a Si MEMS process, which accurately produces a small and thin structure. When a photon pulse from a micro-PMT is amplified by a high-speed preamp, it is possible to obtain a photon pulse of 4-5 ns. Example pulse waveforms from such micro-PMTs show lower noise and distortion than some prior PMTs from smaller input capacitance owing to short distance and small area. In addition, a small photocathode area (e.g., 1×3 mm) can achieve very low dark-count rates at room temperature. According to the velocity distribution of cascade electrons, the photon pulse for the PMT can show a continuous photoelectron (PE) level. The combination of low dark count and narrower pulse width can provide a photon detection system with a wider dynamic range. Some PMTs provide gain and dynamic range control on the outputs for both photocurrent and photon mode, permitting calibrating the detected value to the absolute power level for each measurement condition. In some examples, the output from the SSPMT is a current, and an external transimpedance amplifier (TIA) is used to provide a voltage-based photon signal. In some examples, the SSPMT includes or is packaged with a TIA (e.g., in a system-in-package or system-on-chip configuration) and so can produce a voltage-based photon signal. In various examples such as those shown in FIG. 1, 17, 18, 20, 21A-24 , or 26A-34, in order to collect fluorescence light and couple to the sensor, fiber- or aberration-free optics can be used.

In some examples, an SSPMT does not exhibit a significant quench or recharge time, unlike an SiPM. For example, there is no structure in some SSPMTs that is required to recharge after a pulse. Therefore, some examples that use an SSPMT and not an SiPM do not use differentiation circuitry such as that shown in FIGS. 4A and 5 .

FIG. 33 shows an elevational cross-section of the PMT of FIG. 32 . As shown, the PMT has glass covers on the top and the bottom. In some examples, incoming light may reflect off both the top and the bottom of a glass layer due to the difference in indices of refraction between the outside air, the glass, and the inside environment of the PMT (e.g., vacuum or neutral gas). Similarly, light may reflect off both sides of a protective layer (e.g., ˜3 μm thick) over an Si base used in a SiPM (e.g., FIG. 2C). This can produce interferometric effects that affect SSPD output. Therefore, in some examples, the protective layer, glass, or other layer(s) through which light travels have anti-reflective (AR) coatings or surface structures, e.g., with one or more layers. In some examples, at least one surface of at least one layer in a SiPM or SSPMT has a metamaterial coating or structure, e.g., a surface nanostructure, configured to reduce reflection at that surface.

FIG. 34 shows an example photon-counting measurement system, e.g., using SSPMTs such as shown in FIGS. 32 and 33 . The illustrated components and parameters are illustrative and not limiting. In the case of 10 kcps photons at 405 nm, the intensity is approximately 5 fW. This is roughly 1000 times higher sensitivity than is obtained with a conventional photocurrent approach. Various examples of photon counting can distinguish each photocurrent noise cause, permitting more accurate cellular analysis, e.g., of basic material in flow cytometry and biology. Some examples include optical or other components such as described herein with reference to FIG. 1 . Similar to FIG. 1 , incident light from the laser reaches the sample, and resultant light from the sample is focused by the collection lens into the optical fiber. The optical fiber carries the resultant light (e.g., broadband) to a spectro unit (motor-driven, as shown, or other examples such as those in FIG. 1 ). A photon sensor, e.g., an SSPMT or other SSPD, measures the resultant light at various wavelengths. The illustrated example shows an amplifier, e.g., a TIA, that amplifies the photon signal. Other example spectral discriminators and photon sensors are described herein with reference to Table 1.

FIG. 34 shows autofluorescence (AFL) evaluation optics in a tested example. An excitation wavelength of 405 nm is the shortest wavelength in the visible region with excitation energy that provides a full visible spectrum longer than the laser wavelength. A laser spot 100 μm in diameter illuminates the sample behind a bandpass filter to remove stimulated spontaneous emission in the laser beam. Excited fluorescence photons are collected by an NA-0.125 lens and coupled to an NA-0.22 optical fiber through a 405-nm notch filter and a long-pass filter to remove excitation photons. Detection occurs at 420-900 nm and count is given as total number of photoelectrons (PEs) without detection-efficiency correction. Assuming that excited fluorescence is emitting uniformly to any solid angle, the total number of emitted photons is estimated as 1000 times the measured PE number because of lens collection efficiency ( 1/250) and sensor PDE (¼).

Excited autofluorescence intensity is roughly proportional to illuminating power and sample thickness under fixed optics. In order to compare autofluorescence, an excitation coefficient k is defined as the detected PE number per μW illumination for a 1-mm sample thickness (PEcps/μW mm). Interestingly, quartz, glass, and many materials show AFL and photobleaching. An experiment was performed to check the excitation coefficient before and after photobleaching. Photobleaching is difficult to observe with the noncoherent light source in a conventional fluorescence spectrometer. Lasers can provide very high illumination intensity, over 10⁶ J/m², which is not proportional to total illumination energy (J). Measurement is first dark count, system AFL, and dry vial for liquid, and finally the sample to calculate a count only for that sample.

As an example, an excitation coefficient of 1000 cps/μW mm means that the total number of emitted photon is estimated as 1 Mcps (1 k×1 kcps) under measurement conditions. A photon measurement of 1 Mcps represents about 1 pW of incident flux captured by the sensor. Illumination at 405 nm/1 μW contains 2.04 Gigaphotons. This equals 1 Mcps/2 Gcps, ˜ 1/2000; it takes 2000 illuminating photons to produce a single emitted photon. Using a measured excitation coefficient, it is possible to estimate the AFL from the illumination level. A material with k=1000 cps/μW mm emits 1 k×1 kcps=1 Mcps, ˜1 pW AFL under 1 mW illumination. Several material-evaluation results are shown in Table 3. Table 3 shows the measured excitation coefficient per 1 μW/405 nm exposure and 1-mm sample thickness for basic materials in flow cytometry. Measurements were taken with conditions 405 nm, 1 μW, 100 μm spot, and NA 0.125.

TABLE 3 Excitation coefficient k Material (cps/uW mm) Remark Flow cell quartz 3500-1500 Photobleach before/after Highest grade quartz 700-300 Photobleach before/after Distilled water 250 Tube trapping Sheath 1120 Tube trapping Clean polymer bead 1 pm in 2210 Tube trapping water Clean silica bead 1 pm in water 10,700 Tube trapping Y-G highlighter 1,180,000 Tube trapping Slide glass borosilicate 4700 1 mm thick Cover glass borosilicate 3200 (530) 0.17 mm thick

Many materials for flow cytometry exhibit autofluorescence, e.g., distilled water, sheath, or clean beads. Yellow-Green dye diluted in water for flow check in a cytometer has a count of 1.2 Mcps/μW mm, meaning roughly one emitted photon for every two excitation photons in the illuminated volume. In order to reduce the influence of AFL from the tube, a trapping method involving illumination along the coaxial direction for liquid-sample evaluation can be used, as discussed herein with reference to FIG. 20 .

FIG. 35 shows a silicon photomultiplier in which the anodes of an array of photodiodes are connected to a common ground electrode and the cathodes of the array are connected via current limiting resistors to a common bias electrode for applying a bias voltage across the diodes.

FIG. 36 shows an example SiPM in which a third electrode is capacitively coupled to each photodiode cathode to provide a fast readout of the avalanche signals from the photodiodes. When the photodiode emits a current pulse, part of the resulting change in voltage at the cathode will be coupled via mutual capacitance into the third electrode. Using a third electrode for readout avoids the compromised transient performance resulting from the relatively large RC time constant associated with the biasing circuit.

In the illustrated example, the quench resistors and the fast-readout capacitors are connected to the respective cathodes of the photodiodes. In other examples, the quench resistors and the fast-readout capacitors are connected to the respective anodes of the photodiodes. The illustrated example shows a 1-D arrangement of photodiodes. However, the same circuit can be used in a 2-D array such as that of FIG. 3A or 11A by, e.g., wiring all the cathodes in each row together, and to all the cathodes in the other rows (and likewise for the anodes and the fast output electrodes). In some examples, a 2-D array can be segmented, e.g., into multiple groups of rows, the photodiode circuitry in each group connecting to a single cathode and a single anode, and the groups having respective, different cathodes and respective, different anodes.

In some examples, the readout electrode is insulated by Silicon Oxide or another dielectric from all the elements of the SiPM structure but has weak capacitive coupling to each photocell. Such capacitive coupling provides fast, partial (1-10% or so) injection of the signal charge (which is of the order of 0.15 pC/photocell) when the photocell is firing. The capacitance of the third electrode toward other electrodes of the SiPM stays rather low, of the order of 10% of total SiPM capacitance. To implement this electrode, a transparent conductor (for example very thin gold, or a transparent conducting oxide such as Indium Tin Oxide) or a grid-like structure comprising a non-transparent conductor may be used. The third electrode may be galvanically isolated from the first and second electrode. The signal in the third electrode may be induced electrostatically. The third electrode may be coupled to a cathode of the photodiode, for example via a capacitive coupling. The signal in the third electrode may be induced by capacitive coupling between the third electrode and the photodiode cathodes. The capacitance of the third electrode with the photodiode cathodes may be between 2% and 20% of the total silicon photomultiplier capacitance. The capacitance of the third electrode with the photodiode cathodes may be approximately 10%.

FIG. 37 shows an equivalent circuit showing a manner of operation of various aspects such as that shown in FIG. 36 . The current source I1 represents the avalanche current of the diode, C1 the capacitance of the photodiode and C5 the capacitance between the third electrode and the photodiode. The remaining network represents a biasing and readout circuit. In the illustrated example, the current through R7 simulates the readout, unlike some prior schemes using R3. The current through R7 can respond more quickly in both rise and fall than the current through R3. Various examples include reading the output signal from the third electrode using a wideband amplifier with low input impedance. The wideband amplifier may have a bandwidth greater than 200 MHz and may have an input impedance of less than 200 ohms. Various examples include reading the output signal from the third electrode using a current integrating charge amplifier to provide an output proportional to the number of photodiodes firing within a period of time of interest.

EXAMPLE CLAUSES

Various examples include one or more of, including any combination of any number of, the following example features. Throughout these clauses, parenthetical remarks are for example and explanation, and are not limiting. Parenthetical remarks given in this Example Clauses section with respect to specific language apply to corresponding language throughout this section, unless otherwise indicated.

A: Signal-processing apparatus comprising: a differentiator (e.g., FIG. 1, 4A-6B, or 9) configured to provide a differentiated photon signal based at least in part on a photon signal associated with a photon sensor; and a crossing detector (e.g., FIG. 1, 4A-6B, 7, 9, 10A-10B, or 14) configured to provide a count signal representing a number of times the differentiated photon signal crosses a threshold level.

B: The apparatus according to paragraph A, further comprising a high-pass filter (e.g., FIG. 5 ) configured to receive an output signal of the photon sensor and provide the photon signal.

C: The apparatus according to paragraph A or B, further comprising: a high-pass filter (e.g., FIG. 5 ) configured to receive an output signal of the photon sensor and provide a filtered signal; and a pedestal-clamping circuit (e.g., FIG. 5, 9, 10A-10B, 14 , or 15) configured to determine the photon signal by adjusting a level of the filtered signal based at least in part on an envelope of the filtered signal.

D: The apparatus according to any of paragraphs A-C, wherein the crossing detector further comprises an adaptive envelope reference unit (e.g., FIG. 5 or 7-9 ) configured to provide the threshold level by offsetting an envelope level of the differentiated photon signal by a predetermined offset level.

E: The apparatus according to any of paragraphs A-D, further comprising a silicon photomultiplier (SiPM) (e.g., FIG. 1-4C, 11A-11E, 25, 32, 33 , or 35-37) configured to receive resultant light from a sample and, in response, provide the photon signal.

F: The apparatus according to any of paragraphs A-E, wherein: the apparatus further comprising a silicon-photomultiplier array sensor (FIG. 28 ) configured to receive resultant light from a sample and, in response, provide the photon signal; the array sensor comprises multiple blocks associated with respective sensor channels and arranged along a first direction; and each block (e.g., a single row of pixels, or a two-dimensional array of pixels) comprises a plurality of silicon photomultiplier pixels (e.g., SiPM sensor pixels as in FIG. 2A-4D, 11A-11E, 32, 33 , or 35-37) (e.g., each block can be a freestanding block, or a portion of an area sensor).

G: The apparatus according to paragraph F, further comprising optics (FIG. 29 ) configured to: spread out different wavelengths of the resultant light along the first direction (e.g., a spectral discriminator); and spread out light within a particular wavelength along a second direction substantially different from the first direction (e.g., a beam shaper).

H: The apparatus according to any of paragraphs A-G, further comprising a solid-state photomultiplier (e.g., a micro-photomultiplier tube, μPMT™, by HAMAMATSU, or other SSPMT) configured to receive resultant light from a sample and, in response, provide the photon signal.

I: Signal-processing apparatus comprising: a pulse detector configured to provide a pulse-width signal based at least in part on a photon signal associated with a photon sensor; and a pulse counter configured to provide a count signal based at least in part on both a number of pulses of the pulse-width signal and respective widths of individual pulses of the pulse-width signal (e.g., FIG. 14 or 15 ).

J: The apparatus according to paragraph I, further comprising: a first counting unit configured to count the pulses of the pulse-width signal; a second counting unit configured to count those pulses of the pulse-width signal that have a pulse time greater than a predetermined time (e.g., FIG. 14 or 15 ).

K: The apparatus according to paragraph J, wherein the second counting unit comprises: a comparator (“COMP 2”) configured to provide a signal indicating that a pulse of the pulse-width signal has crossed a predetermined threshold; a delay circuit (“DELAY t0”) configured to enable the comparator substantially the predetermined time after a start of the pulse of the pulse-width signal; and a counter (“Upper Counter”) configured to count outputs of the comparator.

L: A measurement system, comprising: an optical source configured to irradiate a sample; a solid-state photon detector (SSPD) (e.g., a silicon photomultiplier, SiPM, or solid-state photomultiplier tube, SSPMT) configured to receive resultant light from the sample and, in response, provide a photon signal; and a photon counter configured to receive the photon signal and, in response, provide a count signal, wherein the photon counter comprises: a differentiator configured to provide a differentiated photon signal based at least in part on the photon signal; and a crossing detector configured to provide the count signal representing a number of times the differentiated photon signal crosses a predetermined threshold level (e.g., FIG. 1, 17, 18, 20, 21A-21G, 23-25, 28 , or 29) (in some examples, the SSPD can include an anti-reflective coating) (in some examples, optical components of the measurement system can be coated with an optical absorption material, e.g., carbon nanotubes).

M: The system according to paragraph L, wherein: the optical source comprises a laser or other source configured to emit substantially monochromatic light towards the sample; and the system comprises a monochromator (e.g., FIG. 19 or 26A-26B) configured to successively provide a plurality of wavelength bands of the resultant light to the SSPD.

N: The system according to paragraph M, further comprising a drive (e.g., a motor; FIG. 1, 17, 19, 21A-21G, or 25) configured to cause the monochromator to successively provide the plurality of wavelength bands.

O: The system according to any of paragraphs L-N, comprising: illumination optics (e.g., FIG. 1, 17, 18 , or 20) configured to substantially focus the light onto the sample.

P: The system according to paragraph O, wherein: the optical source comprises a laser configured to provide a laser beam having a primary wavelength; the illumination optics comprise a laser bandpass filter upstream of the sample along a direction of travel of the laser beam, wherein the laser bandpass filter is configured to absorb light at wavelengths substantially different from the primary wavelength; the illumination optics comprise a laser notch filter configured to substantially block light at the primary wavelength, wherein the laser notch filter is arranged downstream of the sample along the direction of travel of the laser beam; the illumination optics are configured to focus the laser beam on the laser notch filter; and the illumination optics are configured to focus a beam waist of the laser beam proximal to the sample and the notch filter (e.g., FIG. 18 or 20 ).

Q: The system according to any of paragraphs L-P, further comprising collection optics comprising an optical fiber and configured to convey the resultant light to the SSPD via the optical fiber.

R: The system according to paragraph Q, further comprising illumination optics, wherein the illumination optics, the sample, the collection optics, and the SSPD are arranged in an in-line layout.

S: The system according to paragraph Q or R, wherein: the system comprises a monochromator configured to successively provide a plurality of wavelengths of the resultant light to the SSPD; and the collection optics comprise: a bundle of optical fibers; and a retaining structure configured to arrange the bundle of optical fibers substantially in an elongated shape at an exit of the monochromator and substantially in a circular shape at a port facing the SSPD.

T: The system according to paragraph S, wherein the elongated shape is a substantially rectangular shape having an aspect ratio (long:short) exceeding 3:1.

U: The system according to any of paragraphs L-T, wherein: the optical source comprises a laser or other source configured to emit a beam of light towards the sample; and the system comprises a tube configured to retain the sample and arranged so that the beam does not cross or intersect with the tube within a light-collection area associated with the tube and the sample (e.g., FIG. 20 ) (in some examples, the system can be configured so that the beam does not cross or intersect with the tube).

V: The system according to any of paragraphs L-U, wherein the photon counter comprises signal-processing apparatus as recited in any of paragraphs A-K.

W: A method, comprising detecting resultant light from a sample using detection equipment recited in any of paragraphs A-V.

X: The method according to paragraph W, further comprising irradiating the sample using irradiation equipment recited in any of paragraphs A-V.

Y: A method, comprising: receiving a photon signal representing resultant light from a sample; providing a differentiated photon signal based at least in part on the photon signal; determining a number of times the differentiated photon signal crosses a predetermined threshold level; and providing a count signal representing the number of times.

CONCLUSION

This disclosure is inclusive of combinations of the aspects described herein. References to “a particular aspect” (or “embodiment” or “version”) and the like refer to features that are present in at least one aspect. Separate references to “an aspect” (or “embodiment”) or “particular aspects” or the like do not necessarily refer to the same aspect or aspects; however, such aspects are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to “method” or “methods” and the like is not limiting.

Although some features and examples herein have been described in language specific to structural features or methodological steps, it is to be understood that the subject matter herein is not necessarily limited to the specific features or steps described. For example, the operations of example processes herein are illustrated in individual blocks and logical flows thereof, and are summarized with reference to those blocks. The order in which the operations are described is not intended to be construed as a limitation unless otherwise indicated, and any number of the described operations can be executed in any order, combined in any order, subdivided into multiple sub-operations, or executed in parallel to implement the described processes. For example, in alternative implementations included within the scope of the examples described herein, elements or functions can be deleted, or executed out of order from that shown or discussed, including substantially synchronously or in reverse order.

Each illustrated block can represent one or more operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the operations described herein represent computer-executable instructions stored on at least one computer-readable medium that, when executed by one or more processors, enable the one or more processors to perform the recited operations. Accordingly, the methods and processes described above can be embodied in, and fully automated via, software code modules executed by one or more computers or processors. Generally, computer-executable instructions include routines, programs, objects, modules, code segments, components, data structures, and the like that perform particular functions or implement particular abstract data types. Some or all of the methods can additionally or alternatively be embodied in specialized computer hardware. For example, various aspects herein may take the form of an entirely hardware aspect, an entirely software aspect (including firmware, resident software, micro-code, etc.), or an aspect combining software and hardware aspects. These aspects can all generally be referred to herein as a “service,” “circuit,” “circuitry,” “module,” or “system.” The described processes can be performed by resources associated with one or more data processing systems 3901, 3904 or processors 3986, such as one or more internal or external CPUs or GPUs, or one or more pieces of hardware logic such as FPGAs, DSPs, or other types of accelerators.

Conditional language such as, among others, “can,” “could,” “might” or “may,” unless specifically stated otherwise, are understood within the context to present that certain examples include, while other examples do not include, certain features, elements or steps. Thus, such conditional language is not generally intended to imply that certain features, elements or steps are in any way required for one or more examples or that one or more examples necessarily include logic for deciding, with or without user input or prompting, whether certain features, elements or steps are included or are to be performed in any particular example.

The word “or” and the phrase “and/or” are used herein in an inclusive sense unless specifically stated otherwise. Accordingly, conjunctive language such as, but not limited to, at least one of the phrases “X, Y, or Z,” “at least X, Y, or Z,” “at least one of X, Y or Z,” and/or any of those phrases with “and/or” substituted for “or,” unless specifically stated otherwise, is to be understood as signifying that an item, term, etc., can be either X, Y, or Z, or a combination of any elements thereof (e.g., a combination of XY, XZ, YZ, and/or XYZ). As used herein, language such as “one or more Xs” shall be considered synonymous with “at least one X” unless otherwise expressly specified. Any recitation of “one or more Xs” signifies that the described steps, operations, structures, or other features may, e.g., include, or be performed with respect to, exactly one X, or a plurality of Xs, in various examples, and that the described subject matter operates regardless of the number of Xs present.

This document expressly envisions alternatives with respect to each and every one of the following claims individually, in any of which claims any such reference refers to each and every one of the items in the corresponding group of items. Furthermore, in the claims, unless otherwise explicitly specified, an operation described as being “based on” a recited item can be performed based on only that item, or based at least in part on that item. This document expressly envisions alternatives with respect to each and every one of the following claims individually, in any of which claims any “based on” language refers to the recited item(s), and no other(s). Additionally, in any claim using the “comprising” transitional phrase, a recitation of a specific number of components (e.g., “two Xs”) is not limited to embodiments including exactly that number of those components, unless expressly specified (e.g., “exactly two Xs”). However, such a claim does describe both embodiments that include exactly the specified number of those components and embodiments that include at least the specified number of those components.

High-speed wireless communication has been put into practical use in the air and in outer space, but in environments such as seawater, high-density water molecules and other factors such as electrical conductivity of dissolved ions, long-distance communication using electromagnetic waves in the sea has proven to be very difficult, thus low transfer rate acoustic communication using low frequency waves has been the mainstay of underwater communication, which is prone to eavesdropping. Thus, high-speed wireless communication in the sea and underwater can be said to be an unexplored area.

There is a growing need for high-speed wireless communication for transmission and control of underwater moving vehicles in a three-dimensional (3D) space. Optical wireless communication using light such as LEDs and laser light is expected as a high-speed communication technology in the sea, but it is difficult due to absorption characteristics of seawater and organic particles. Due to attenuation and scattering, even if a watt-class laser and a highly sensitive sensor such as a photomultiplier tube are used, these systems can only reach about 100 meters between a transmitter and a receiver. Also, in order to perform two-way communication in the three-dimensional space of water, a transmitter and a receiver must include tracking technology that aligns the position on a straight line. However, with conventional vacuum tube sensors, it was challenging to configure a split-type sensor capable of measuring beam shift. This present disclosure discloses a sensor package that includes a large number of Geiger mode avalanche diodes which represent a solid-state device, to detect single photons, and thus significantly improving dead time of the systems disclosed in the prior art by at least a factor of 100 in terms of communication speed. Specifically, the dead time of the photodetection is improved to about 0.1 ns, which is at least 100 times faster than the highest found in the prior art. Additionally, by employing a novel alignment technology, and thus achieving longer distances possible between the receiver and the transmitter. Accordingly, the luminous flux distribution is detected by a split type sensor and tracking in 3D space. The result is to enable longer distance communication in underwater environment.

As discussed above, water presents the lowest absorption to light in the blue-green wavelength range. Referring to FIG. 38 , a graph of absorption coefficient in cm⁻¹ vs. wavelength is provided. The absorption coefficient describes how far light of a particular wavelength can travel into a material before being completely absorbed. FIG. 38 shows a marked drop in electromagnetic attenuation in water in the blue-green range. Consequently, a blue-green laser as the source of light and thus photons realizes the least amount of attenuation in water, thus maximizing range of communication between the transmitter and the receiver.

Photomultiplier tubes (PMTs) have been used as high-sensitivity photosensors for more than half a century, but as vacuum tubes. There are restrictions such as size and magnetic influence. In recent years, silicon photomultiplier (SiPM) using Geiger mode (avalanche effect) has been developed and commercialized as a single photon sensor with characteristics such as small size, low bias, and high sensitivity. On the other hand, SiPM requires a process of several tens of nanoseconds to recharge electrons after an avalanche process in picoseconds, which is referred to as dead time. In the case of high-speed communication, the dead time limits the communication rate. A significant contribution of the present disclosure is the improvement in dead time as compared to what has been reported in the prior art (namely the best dead time sensor reported in the prior art can allegedly achieve 10 ns resulting in a communication speed of 100 Mbps (mega bits per second), whereas the novel system disclosed herein can achieve a dead time of 0.1 ns, representing a 100× improvement in communication speed). Towards this end, the present disclosure described an underwater sub-nanosecond photoelectronic signal detection system utilizing the avalanche signal from each photon with giga-band signal processing. In addition, the thermos-electron dark count was reduced more than two digits compared to room temperature by thermoelectric cooling. With the sensor package and the accompanying hardware and software system disclosed herein, it is possible to detect the photocurrent and photon signal at the same time. The sensor package is disclosed can be divided into four elements and obtain the matrix output to measure the luminous flux intensity distribution on the sensor as part of the novel alignment scheme, to thereby control spot position using the four-element sensor while achieving single-photon level detection and control.

The sensor package provided in FIG. 4A shows an example circuit using a differentiator (represented as an RC high-pass filter followed by a buffer (triangle)) to provide 1 ns photon pulses and thus suitable for capturing the lifetime emission of the fluorescent molecules. As discussed above, the SiPM output waveform has a fast avalanche process measured in picoseconds and a slow recharge process measured in nanoseconds. The high-pass filter and signal differentiation permit detecting only the avalanche process. In the illustrated example, the differentiator's input (on capacitor C) is connected opposite the quench resistor from the APD.

The sensor package shown in FIG. 4A, redrawn in FIG. 39A, is configured to be coupled to the circuit shown in FIG. 7 , as discussed above. It should be appreciated that the sensor package shown in FIG. 39A includes high pass filter capacitors as part of the sensor package. In other words, each Geiger mode avalanche diode is coupled to a quench resistor and a high pass filter capacitor internally. A dashed box around the sensor package represents the boundary of the sensor package with external pins shown with dashed circles. The sensor package may include a large number of such components, in the order of 100s to 1000s. All high pass capacitors are coupled to each other and coupled to one of the three external pins. A resistor coupled to the common capacitor point (i.e., external pin of the sensor package) is coupled to the ground which forms the high pass filter in connection with the internal capacitors. It should be appreciated that the sensor package may present the high pass capacitor as external components, in which case one or more capacitors would be presented in coupling with the resistor to form the high pass filter. The output of the high pass filter is presented to a single stage amplifier which is then presented to a comparator as described in further detail with reference to FIG. 39B, generating pulses associated with each photon striking the sensor package, and a counter to count the pulses. Another of the three external pins of the sensor package is coupled to the Vbias; and the third external pin, representing a common point for all the Geiger mode avalanche diodes, is coupled to a single stage transimpedance amplifier (TIA) which generates the TIA output further discussed in the circuit of FIG. 39B. The graphs provided in FIG. 39A represent interim waveforms prior to being further processed as shown in FIG. 39B. The two outputs shown in FIG. 39A are GHz D-Out which is an output of the photon detector that presents the photoelectron response of an incident photon in the detector. Each detected photon is converted into a differentiated photoelectron (PE) pulse with widths between 400 ps to 600 ps (or about 560 ps as the mean). I-V N-Out is an output of the simultaneous detection of a photon and photocurrent. As discussed above, the sensor package is cooled via a thermoelectric cooling approach. Accordingly, various examples provide further reduction of the dark count by actively cooling the sensor package, e.g., using a cooling unit. Various examples include a Peltier cooling unit and can achieve dark counts lower than e.g. 1 kcps, or about 100 cps at −30° C., with 10 cps standard deviation.

Referring to FIG. 39B, a schematic is shown depicting how to couple the circuit in FIG. 39A to the schematic shown in FIG. 7 , via a simplified version of the circuit shown in FIG. 5 , as described above. Specifically, the Geiger mode APDs in the sensor package shown in FIG. 4A or FIG. 39A are coupled to a single-stage amplifier with a gain, e.g., 50 dB (i.e., the output power of the amplifier is the input power*50). The output of the single-stage amplifier is then coupled to the positive terminal of the comparator via a delay line. The output of the comparator is provided to a counter to count pulses which is then provided to software for decoding by a decoder. Alternatively, the output of the single stage amplifier may be provided to an analog-to-digital converter (ADC₂) thereby converting the amplified analog signal to a digital signal and provide that digital signal to a central processing unit which can then apply an adaptive threshold comparison by a software-based comparator as discussed below to generate pulses followed by a software-based counter to count said pulses and then followed by a decoding function by a decoder. At the same time the third external pin of the sensor package of FIG. 4A or FIG. 39A is also routed through a single-stage transimpedance amplifier (TIA) thus generating a TIA output. The TIA output is low-pass filtered and provided to an analog-to-digital converter (ADC₁) whose output is provided to an integrator, a peak detector, and a pulse width measurement block. The comparators (hardware-based in FIGS. 39A and 39B and software-based in FIG. 39B) all operate on the basis of adaptive thresholds. This means the input to the delay line is used after being delayed on the positive terminal of the hardware-based comparator and compared to a threshold that is adaptive based on the input to the delay line, as well as the software-based comparator, as discussed below. The adaptive threshold is obtained via an envelope detection circuit, i.e., the same input that is fed to the delay line is fed to an enveloped detection circuit to determine the envelope on the negative side of said signal. This envelope is band passed filtered to remove low and high frequency noise (in both hardware-based and software-based approaches) and is then level shifted by a constant value provided by a digital-to-analog converter (DAC), in hardware-based approach or added to the raw signal as discussed below in software-based approach. The predetermined value of the DAC is chosen so that it is sufficiently large so as to avoid noise on the signal but not so large so as to miss legitimate peaks. The level-shifted band-pass filtered signal is input to the negative terminal of the comparator, while the delayed signal is provided to the positive terminal of the comparator, in the hardware-based approach. The software-based approach is discussed in detail below.

Reference is alternatively made to FIGS. 39B and 40A to demonstrate the hardware-based operation of the circuit shown in FIG. 39B. The thick dashed black line in FIG. 40A represents the raw input to the delay line. The red solid line represents a negative envelope of the dashed black line (i.e., the input to the delay line). The dashed red line represents the level-shifted envelope by the analog value of the DAC. The dotted line pulses represent the comparator output.

In an alternative approach, carried out by software or corresponding hardware, a constant threshold may be applied to a level-shifted version of the raw signal (i.e., the output of the ADC₂), as demonstrated in FIGS. 40B and 40C. For example, in FIG. 40B, the blue solid line represents the dashed black line (the output of ADC₁) level-shifted by the red solid line (represents a negative envelope of the dashed black line), so that the blue line has a minimum value of 0 V. In FIG. 40C, the blue line is repeated but with a constant predetermined threshold. The constant predetermined threshold again represents a noise floor (i.e., the threshold is sufficiently high to block noise), and sufficiently low so as to detect all legitimate peaks. In this case, the threshold is about 0.05 V. The threshold is used with a software-based comparator to provide pulses.

These two alternative approaches provide the same results. To depict the similarity between these two approaches, reference is made to FIG. 40D, where a portion of the signal in FIGS. 40B and 40C are shown applying both approaches. FIG. 40D shows the peak detection of a photoelectron PE stream by local maxima of the signal. Again, the solid red line represents the negative envelope and the dotted red line represents a level-shifted version of the negative envelope raised by the predetermined value of the DAC. At the same time, the solid blue line represents a level-shifted version of the dashed black line by the envelope. In the first instance, the dashed black line is compared to the dotted red line generating a series of pulses shown in thin dashed lines. In the second instance, the blue solid line is compared to a constant threshold and as it is shown which produces the same pulse train. In both of these alternative approaches, the threshold is considered to be adaptive (i.e., based on the input signals). This is true, even though the threshold in the second instance is constant, since the input signal has been affected by the envelope. The bit stream shown in FIG. 40D, which can be obtained from both a hard-ware-based solution and a software-based solution is a detected and decoded bit stream based on a predetermined timing sequence.

Referring back to FIG. 40C, the peaks are determined by using three separate criteria: 1) the signal must be higher than a minimum threshold to avoid acceptance of noise (the value of the threshold shown in FIG. 40C is 0.05 V, according to one embodiment); 2) peak width must be greater than a predetermined threshold, e.g., 500 ps; and 3) relationship to neighboring peaks (e.g., if there is a peak that is all by itself, that peak may be considered noise). The latter is referred to herein as the prominence of the peaks which is related to neighboring peaks.

Referring to FIG. 40E, the advantage of adaptive threshold is shown by depicting how without the adaptive threshold dynamic range of the comparator plateaus at about 5.0E+08 cycles per second, whereas with the adaptive threshold, the dynamic range continues to increase without showing a plateau, at least up to 8.0 E+08 cycles per seconds. Specifically, FIG. 40E provides a comparison of the number of counts per second acquired by the photon detector or photomultiplier between comparator with and without adaptive threshold schemes.

As discussed above, since sensitivity of the optical detectors is of paramount concern, alignment between the transmitter and receiver is critical. To ensure such alignment, a closed loop feedback system is implemented based on error signals generated by photodetectors which is thus used to control a beam steering device of the transmitter. In order to detect main channel signal and beam profile simultaneously and effectively, 4-element photon sensor arrays are used. Referring to FIG. 41 , which is a schematic for a beam position control system, beam position signal is calculated by A, B, C and D element matrix signal. Specifically, the beam position control system includes 4-element photon sensor arrays at the transmitter and at the receiver. Each element in the 4-element photon sensor is a Geiger mode avalanche diode producing a voltage associated with how much of the photon has struck the associated detector. Therefore, if there is no error, i.e., the photon is at the center between all four elements, the voltage associated with each element is equal to the voltage associated with the other three elements. In other words, suppose V_(A) represents the voltage associated with element A, V_(B) represents the voltage associated with element B, V_(C) represents the voltage associated with elements C, and V_(D) represents the voltage associated with element D, as shown in FIG. 39 . If the photon strikes the 4-element photon sensor array exactly in the center, V_(A)=V_(B)=V_(C)=V_(D). If, however, the photon strikes the 4-element photon center array off its center position, say most of the photon is positioned in the element A, while the other elements receive equal optical energy (thus, V_(A)>>V_(B)=V_(C)=V_(D)), then the X-axis error can be calculated based (V_(A)+V_(D))−(V_(B)+V_(C)), and the Y-axis error can be calculated based on (V_(A)+V_(B))−(V_(C)+V_(D)). These axes-error signals can be provided to a controller each as a feedback signal to correct alignment between the transmitter and the receiver using an optimization process (e.g., means squared error, if done in software) until each of the two errors is below a predetermined threshold. The 2-axis MEMS mirror device is appropriate for rapid and accurate beam steering. In FIG. 41 , a transmitter of a first transceiver and a receiver of a second transceiver are shown at each end of the optical communication channel. It should be appreciated that each of the transceivers includes a decoder and an encoder to decoder messages received and encode messages to be sent. A number of encoder/decoder schemes are known to a person having ordinary skill in the art, and not described herein.

Alternatively, the feedback control can be accomplished in hardware as provided in FIG. 42 , which is a schematic of proportional-integral-derivative (PID) controller with a feedback path. The PID controller shown in FIG. 42 includes a controller schematic with a feedback loop. The output in time domain is expressed as:

${u(t)} = {{K_{p}{e(t)}} + {K_{i}{\int{{e(t)}{dt}}}} + {K_{p}\frac{de}{dt}}}$

The transfer function of the PID is determined by found by obtaining the Laplace of the above equation which is defined as:

${K_{p} + \frac{K_{i}}{s} + {K_{d}s}} = \frac{{K_{d}s^{2}} + {K_{p}s} + K_{i}}{s}$

-   -   where, K_(p) is the proportional gain,     -   K_(i) is the integral gain, and     -   K_(d) is the derivative gain, as known to a person having         ordinary skill in the art. Referring to FIG. 43 , graphs of         attenuation coefficient (1/m) vs. wavelength (nm) are provided         which can be used to depict the attenuation vs. distance (m)         based on various absorption coefficients (e.g., 0.01, 0.025, and         0.05). In the PID controller diagram shown in FIG. 42 , the         block that is typically shown in a PID diagram and referred to         as the “Plant” is shown as the XY alignment block which includes         servos in both transceivers to align the light.

It should be appreciated that while FIGS. 38 and 43 show the best wavelength for attenuation to be around blue-green color, thus utilizing blue-green laser as the light source for maximum strength, i.e., least attenuation, other wavelengths, e.g., in the ultraviolet and infrared regions, can also be used.

Referring to FIG. 44 , an optical schematic is shown at each transceiver with a light detection and ranging (LIDAR) function, known to a person having ordinary skill in the art. LIDAR represents a scheme that uses pulsed laser to measure distance (including time of travel from a transmitter, e.g., of transceiver number 1, to an object, e.g. the transceiver number 2, and back to the transmitter representing time of flight*2, which can be used to determine distance between the transmitter and the object by multiplying the speed of light in the medium by the overall time of flight (i.e., between the transmitter, the object, and back to the transmitter) divided by 2). Underwater photon communication optics includes high speed modulation laser diode (LD) with an accompanying lens thereby providing a modulated laser output which is provided to a polarization beam splitter (PBS) which routes the modulated laser output to i) a transmitter photon sensor, and ii) to a quarter wave plate (λ/4) thus generating a quarter wave polarized output. The quarter wave polarized output is then provided to a beam steering actuator to control beam angle to one or more lenses to modify the laser beam diameter for outputting out of the transceiver. The transceiver also includes a 2×2 photon sensor to detect modulated signal and beam direction. The two transceiver can apply the same wavelength or different wavelengths. In the case of same wavelength, the transceivers use polarization to maximize detection signal, while use of 2 wavelengths can lead to avoidance of crosstalk between what is transmitted and what is received.

Referring to FIG. 45 , a graph of beam diameter (expressed as spot size) in mm vs. distance also in mm is provided. In order to maximize detection signal, a collimated beam is used by the transmitter portion of each receiver is effective, utilizing a collimator lens. A collimator lens. A collimating lens is typically constructed using a tube with one or more lenses disposed therein which can generate the proper focal distance. A collimator lens can be directly coupled to the source of light, e.g., a laser, or be coupled via an optical fiber. The beam size is expressed by:

${w(z)} = {w_{0}\sqrt{{1 + \left( \frac{z}{z_{R}} \right)^{2}},}}$

where:

${z_{R} = \frac{\pi w_{0}^{2}n}{\lambda}},$

-   -   where w(z) is radius at which the field amplitude drops to 1/e         and field intensity to 1/e² of their axial values, respectively,     -   w₀ is the waist size,     -   z is axial distance from the beam's narrowest point,     -   λ is the wavelength of a given laser, and     -   n is the medium's characteristic impedance.

The beam diameter and detection intensity represent a tradeoff. Larger diameter at longer distances reduce detection intensity. Assuming a maximum distance at 300 m, beam diameter of 50 mm originating from the transmitter of the first transceiver provides almost the same diameter at the receiver of the second transceiver. Example transmitter beam diameters of 10 cm and 20 cm provide distance of 1000 m and 2000 m, respectively. Therefore, for general underwater communication, a beam diameter greater than 50 mm is desired.

Referring to FIG. 46 the above-discussed quadrant sensor for XY alignment scheme is further depicted. A 2×2 photon sensor can detect incident beam direction utilizing a matrix signal on X & Y axes. Position shift is proportional to lens focal length and incident angle (rad). This type of matrix signal is widely used for optical drive optics with PIN photocurrent signal. However, to the best knowledge of the inventors of the present disclosure, a photon base matrix signal generation has never been realized. However, with the advantage of a single photon-based detection scheme disclosed herein such a photon base matrix signal generation is achievable.

Referring to FIG. 47 , a graph of attenuation distance based on Beer-Lambert law is provided. The light power in the seawater is described based on: I=I ₀ e ^(−c(λ)z),

-   -   where I and I₀ represent the power of received and transmitted         light, respectively,     -   z is the transmitted distance, and     -   c(λ) is the attenuation coefficient at wavelength λ of the         light. In order to achieve a goal of 300 m distance for         underwater communication, attenuation coefficient of less than         0.1 is required. Thus, for c=0.1 one can achieve 0.1 pW at 300 m         or for c=0.0.5 one can achieve 0.1 pW at 600 m depending on the         attenuation coefficient which is dependent on particulates in         water. Inspecting FIG. 47 , it can be seen that emitted laser         intensity is decreased by water attenuation and scattering         determined by Beer-Lambert law. Attenuation coefficient is about         0.1 for clean ocean. This means 1 W laser power is decreased to         0.1 pW at 300 m distance. Even at 0.1 pW, there is sufficient         photons to achieve 220 kbps at 450 nm.

Referring to FIG. 48 , a graph of laser photon number vs. laser power is provided for a laser having a wavelength of 450 nm, utilizing different technologies. In order to reproduce square waveform by photocurrent, over 1000 photons are required. Photon detection by ON-OFF keying is possible to reproduce a bit by about 10 photons. Thus, detected intensity defines user transfer rate. It is possible to communicate even with limited number of photon (i.e., 10 photons per pulse). Thus, the single-photon system disclosed herein advantageously extend communication distance by detecting single photons.

Referring to FIG. 49 , a schematic of distance measurement scheme for underwater optical communication applications according to the present disclosure is provided. It should be appreciated that initially the distance between the two transceivers is unknown, which such uncertainty can be deleterious to underwater communication. Light velocity in water is expressed as light velocity in vacuum divided by water refractive index (1.33) which results in a light velocity in water of 2.25E+8 m/s. One way to measure distance is by LIDAR. Measured distance is based on light velocity in water multiplied by Δt/2 (where Δt is time between when the transmitter sends a pulse, bounces off the receiver and is received back at the transmitter; alternatively, if the receiver and the transmitter of the two transceivers can be synchronized to a common clock, then Δt can be represented by one leg of transmission, where the distance is determined by speed of light in water multiplied by Δt). For example, 1 us time difference is equal to 2.25E+8*1E-6/2 or 112 m.

Referring to FIG. 50 , a schematic of raster scanning is presented, which shows two transceivers 1 and 2, where transceiver 1 can detect direction and distance of transceiver 2 by reflective beams. As discussed above, collimated beam has an advantage of large signal intensity at longer distances. On the other hand, it is difficult to ascertain transceiver position due to the small area presented by collimated beams. Raster scan of collimated beam allows the system to determine transceiver direction and capture by a closed loop servo loop at reflected beam. Once the distance and direction are ascertained, precise position control and stable photon communication can be achieved.

Closed Loop Servo Control has Never Applied to Underwater Communication

It should be emphasized that many variations and modifications can be made to the above-described examples, the elements of which are to be understood as being among other acceptable examples. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. Moreover, in the claims, any reference to a group of items provided by a preceding claim clause is a reference to at least some of the items in the group of items, unless specifically stated otherwise. 

The invention claimed is:
 1. An optical underwater communication system, comprising: a first transceiver and a second transceiver, each of the first and second transceivers including: one or more optical sources configured to provide light activated and deactivated according to a first bit stream, one or more sensor packages each comprising a plurality of photodetectors configured to receive light from another of the first and second transceivers and, in response, provide i) an output voltage signal corresponding to photoelectron response of an incident photon on the one or more sensor packages, and ii) an output current signal in form of peaks corresponding to photoelectron response of an incident photon on the one or more sensor packages, a detector configured to i) convert the output voltage signal and the output current signal to pulses associated with arrival of photons, and ii) count the number of pulses based on a predetermined timing sequence, an encoder configured to encode a message to be sent into the first bit stream, according to a predetermined communication protocol that is based on the predetermined timing sequence, and a decoder configured to decode a message received into a second bit stream, according to the predetermined communication protocol that is based on the predetermined timing sequence.
 2. The optical underwater communication system of claim 1, wherein each of the plurality of the photodetectors is a Geiger mode avalanche photodiode (APD).
 3. The optical underwater communication system of claim 1, wherein each of the one or more optical sources is a laser.
 4. The optical underwater communication system of claim 1, wherein the first bit stream and the second bit stream each has an activation time interval between 1 ns and 100 ns.
 5. The optical underwater communication system of claim 1, wherein the predetermined timing sequence represents a dead time associated with the one or more sensor packages of about 0.1 ns, thereby representing a communication frequency of about 10 GHz.
 6. The optical underwater communication system of claim 1, wherein the output voltage signal of each of the one or more sensor packages is compared against an adaptive threshold to generate corresponding pulses.
 7. The optical underwater communication system of claim 6, wherein the adaptive threshold is established based on a hardware-based solution.
 8. The optical underwater communication system of claim 6, wherein the adaptive threshold is established based on a software-based solution.
 9. The optical underwater communication system of claim 8, wherein the emission pulses are interrogated based on a plurality of criteria applied to the output current signal including i) a predetermined threshold, ii) measurement of peak widths of each peak; and iii) relationship of each peak to neighboring peaks, to determine whether a peak is legitimate.
 10. The optical underwater communication system of claim 1, wherein the detector further configured to align the one or more sensor packages of the first transceiver and the second transceiver to thereby maximize communication distance.
 11. An optical underwater communication method, comprising: activating and deactivating light by one or more optical sources according to a first bit stream by a first transceiver and a second transceiver; each of the first and second transceivers receiving light from another of the first and second transceivers by one or more sensor packages each comprising a plurality of photodetectors and, in response, provide i) an output voltage signal corresponding to photoelectron response of an incident photon on the one or more sensor packages, and ii) an output current signal in form of peaks corresponding to photoelectron response of an incident photon on the one or more sensor packages; converting the output voltage signal and the output current signal to pulses associated with arrival of photons by a detector; counting the number of pulses based on a predetermined timing sequence by the detector; encoding a message by an encoder to be sent into the first bit stream, according to a predetermined communication protocol that is based on the predetermined timing sequence; and decoding a message by a decoder into a second bit stream, according to the predetermined communication protocol that is based on the predetermined timing sequence.
 12. The method of claim 11, wherein each of the plurality of the photodetectors is a Geiger mode avalanche photodiode (APD).
 13. The method of claim 11, wherein each of the one or more optical sources is a laser.
 14. The method of claim 11, wherein the first and the second bit streams each has an activation time interval between 1 ns and 100 ns.
 15. The method of claim 11, wherein the predetermined timing sequence represents a dead time associated with the one or more sensor packages of about 0.1 ns, thereby representing a communication frequency of about 10 GHz.
 16. The method of claim 11, wherein the output voltage signal of each of the one or more sensor packages is compared against an adaptive threshold to generate corresponding pulses.
 17. The method of claim 16, wherein the adaptive threshold is established based on a hardware-based solution.
 18. The method of claim 16, wherein the adaptive threshold is established based on a software-based solution.
 19. The method of claim 18, wherein the emission pulses are interrogated based on a plurality of criteria applied to the output current signal including i) a predetermined threshold, ii) measurement of peak widths of each peak; and iii) relationship of each peak to neighboring peaks, to determine whether a peak is legitimate.
 20. The method of claim 11, further comprising: the detector aligning the one or more sensor packages of the first transceiver and the second transceiver to thereby maximize communication distance. 